What size strap does the Alpina Seastrong Diver 300 Heritage take?

21mm at the lugs, on every reference of the 42mm Diver 300 Heritage. Alpina's own product pages print the field directly: the US site carries Lug Width 21 mm with Band Length 254 mm, and the global site prints Strap Lock Width 21 with a Buckle Size of 18mm on an 80 x 120mm rubber strap. So the strap order is 21mm at the case, tapering to an 18mm buckle. The hands-on reviews and the measured reference tables all agree on 21mm. The trap is assuming the bigger Seastrong diver width: the 44mm Seastrong Diver 300 takes 22mm, not this watch.

Why does my Alpina Diver Extreme need a 16mm strap?

Because the Seastrong Diver Extreme is not a big lug-width diver at all. It is a 39 x 40.5mm cushion case with an integrated strap mount, and Alpina's own spec fields print Strap Lock Width 16 for the case and a 15.25mm strap lock size on the rubber strap. Independent dealer listings measure the same 16mm. A generic 22mm or 20mm strap does not fit: the strap ends are shaped to the case, so replacements come from Alpina's own 16mm accessories or strap makers who list the Diver Extreme explicitly. The 16mm figure that looks wrong for a diver is in fact one of the few places where the brand's own field and every measurement agree.

Do all Alpina Seastrong watches take the same strap?

No, and that is the main Alpina ordering trap. The 44mm Seastrong Diver 300 takes 22mm. The 42mm Seastrong Diver 300 Heritage takes 21mm. The 44mm Seastrong Diver Gyre takes 22mm while its 36mm Comtesse sibling takes 18mm. And the Seastrong Diver Extreme takes a 16mm integrated strap that no standard replacement matches. Two watches both named Seastrong Diver 300, one generation apart, differ by a full millimeter, so the reference on the caseback, not the collection name, decides the strap.

What lug width is the Alpina Alpiner Extreme?

16mm on the 41 x 42.5mm case, and 15.25mm on the smaller 39 x 40.5mm case, both straight from Alpina's Strap Lock Width spec field. Like the Seastrong Diver Extreme it shares the design with, the mount is integrated: the rubber straps and the steel bracelet are shaped to the case, so the width number alone does not make a generic strap fit. One large dealer prints 18mm for one Alpiner Extreme Automatic reference, a divergence this chart flags rather than averages; the brand's own pages say 16mm for that case, and the regulator models measure 16mm at the same dealer.

Alpina band size chart by model

No current brand spreads one collection across as many strap widths as Alpina. The Seastrong Diver 300 is a 44mm watch on 22mm lugs ^[10] ^[12], the Seastrong Diver 300 Heritage is a 42mm watch on odd 21mm lugs ^[1] ^[6], and the Seastrong Diver Extreme is a 39mm cushion whose strap mounts at 16mm ^[20] ^[22], a number that looks like a misprint until you see the case. Alpina prints the answer on its own product pages, a Lug Width field on the US site and a Strap Lock Width field on the global site, and this chart is anchored on those fields reference by reference, with a second independent surface on every row ^[5]. The three traps that generate wrong Alpina strap orders: assuming one Seastrong width fits all, buying a generic strap for an integrated-mount Extreme, and rounding the 21mm Heritage up to 22mm because 21 is thin on the ground at strap shops ^[9].

Quick answers: Seastrong Diver 300 (44mm) takes 22mm. Seastrong Diver 300 Heritage (42mm) takes 21mm, buckle 18mm. Seastrong Diver Gyre (44mm) takes 22mm; Gyre Comtesse (36mm) takes 18mm. Seastrong Diver Extreme takes a 16mm integrated mount (15.25mm strap). Alpiner Extreme 41mm takes 16mm integrated; the smaller 39mm case takes 15.25mm. Alpiner 4 Automatic (40mm) takes 20mm. Alpiner Date Manufacture takes 21mm. Heritage Carree takes 22mm. Startimer Pilot Automatic is 21mm (Alpina prints 21.15mm), Pilot Heritage 23mm, Pilot Chronograph 22mm.

Two Seastrong divers, one generation apart

The name collision at the center of this chart: the Seastrong Diver 300, the cushion-cased 44mm automatic introduced in 2019, and the Seastrong Diver 300 Heritage, the 42mm vintage-styled reissue of the 1960s Seastrong 10. They share a name, a depth rating and a family, and they do not share a strap: the 44mm Diver 300 measures 22mm at the lugs on dealer spec sheets and owner reviews ^[10] ^[13], while the Heritage prints Lug Width 21 mm on Alpina's own US product page and Strap Lock Width 21 on the global one ^[1] ^[2]. Alpina's current catalog pages for the 44mm model no longer print a width field, so that row below stands on dealer and press measurements, disclosed as such, while the Heritage row stands on the brand's own field. One strap guide prints the Diver 300 at 18mm, a figure that matches nothing else on the record and is named here as an error rather than charted ^[15].

The Diver Extreme is where intuition fails completely. It sounds like the biggest watch in the line; it is actually the smallest, a 39 x 40.5mm cushion that replaced the 41mm case when the Extreme design entered the Seastrong collection ^[24]. Its strap does not span standard lugs at all: Alpina's spec block prints Strap Lock Width 16 for the case and a 15.25mm strap lock size on the fitted rubber ^[20], and the dealer that stocks it measures 16mm lug width on three references ^[22]. When a measured-reference site lists the Diver Extreme at 16mm ^[5], it is not a typo; the specs-aggregator that prints 20mm for the same watch is the typo ^[25].

The chart: 14 verified rows

Every row carries at least two independent surfaces: Alpina's own product pages (the Lug Width field on us.alpinawatches.com, the Strap Lock Width and Strap Lock Size fields on alpinawatches.com), the brand's own straps-and-bracelets catalog, measured-reference strap databases, dealer spec sheets, or hands-on reviews with printed measurements. Widths are the spring-bar or integrated-mount width at the case, the first number of a strap listing. Where surfaces disagree, the disagreement is printed, not averaged.

Alpina band width by model family and reference
Model	Case	Lug width	Strap notes
Seastrong Diver 300 Heritage (42mm, all refs incl. AL-520GR4H6, AL-525S4H6, AL-520BY4H6)	42 mm	21 mm	the brand's own field on both sites: Lug Width 21 mm / Strap Lock Width 21, buckle 18mm, rubber 80 x 120mm, band length 254mm assembled; the reviews and measured tables agree, and the strap shops understock 21mm [1] [2] [3] [4] [6] [8]
Seastrong Diver 300 Automatic (44mm cushion, AL-525x4TV6 / V4 / V6 class, incl. GMT)	44 mm	22 mm	the one Seastrong row without a live brand width field, disclosed: 22mm on three dealer spec sheets and two owner reviews, 49mm lug to lug; not 21mm like the Heritage it shares a name with [10] [11] [12] [13] [14]
Seastrong Diver Gyre Automatic (44mm, AL-525LBN4VG6 / AL-525LNSB4VG6)	44 mm	22 mm	22mm on the dealer listings for both limited references, 50mm lug to lug, on the measured table and the launch specs; ships on a recycled rPET NATO with an extra apple-leather strap, both 22mm [5] [16] [17] [18] [19]
Seastrong Gyre Comtesse / Seastrong Automatic (36mm ladies, AL-525LBN3VG6)	36 mm	18 mm	the 36mm Gyre sibling measured at 18mm on the strap database under both its Comtesse and Seastrong Automatic names, matching the ladies-model launch coverage; one millimeter wider than most women's Alpina straps of the class [5] [18]
Seastrong Diver Extreme Automatic (39mm cushion, AL-525K3VE6 / B3VE6 / N3VE6)	39 x 40.5 mm	16 mm	integrated mount: Alpina prints Strap Lock Width 16 with a 15.25mm strap lock size on the 75 x 115mm rubber; the stocking dealer measures 16mm lug width on three references; generic 20/22mm straps do not fit [20] [22] [23] [5]
Seastrong Diver Extreme Automatic GMT (39mm cushion, AL-560LG3VE6 / AL-560B3VE6)	39 x 40.5 mm	15.25 mm	the GMT prints 15.25 in both the case field and the strap lock field on Alpina's page, with the measured table rounding to 16mm; same integrated mount, same non-generic rule as the three-hander [21] [5] [20]
Alpiner Extreme Automatic and Chronograph (41mm cushion, AL-525N4AE6 / B4AE6, AL-730NS4AE6B)	41 x 42.5 mm	16 mm	Strap Lock Width 16 with 16mm strap lock size on Alpina's pages for the automatic and the chronograph; the regulator measures 16mm at the dealer; one dealer page prints 18mm for a black-dial reference, flagged below, not averaged [26] [28] [29]
Alpiner Extreme Automatic, smaller case and Solarmetre (39mm, AL-525BG3AE6 class, AL-140W3AE6)	39 x 40.5 mm	15.25 mm	the smaller Extreme case prints Strap Lock Width 15.25 on the brand page, and the measured table's one 15mm row is the Solarmetre of the same case family; a quarter-millimeter oddity that only brand-shape straps and the brand's own fittings match [31] [32]
Alpiner 4 Automatic (40mm, AL-525NS4AQ6B)	40 mm	20 mm	Strap Lock Width 20 and 20mm strap lock size on the brand page for the Glacier Blue reference, with the retail writeup carrying the matching 40 x 46.5mm case dimensions; the even width makes this the easiest Alpiner to strap-shop [33] [34]
Alpiner Date Manufacture (41.5mm, AL-706)	41.5 mm	21 mm	Strap Lock Width 21 with an 18mm buckle on the brand page, and the specs aggregator independently lists 21mm for the model; the manufacture-movement dress-sport watch on the same odd width as the Diver Heritage [35] [25]
Heritage Alpiner Carree Automatic (rectangular, incl. 140 Years AL-530BA3C6 and Edition Europe AL-530BBG3C6)	32.5 x 39 mm	22 mm	Strap Lock Width 22 on both brand pages, 22mm at the dealer spec sheet and the launch coverage; a 22mm strap on a 32.5mm-wide rectangular case is the brand's widest ratio, see the Carree section for the buckle disagreement [36] [38] [39]
Startimer Pilot Automatic (41mm, AL-525BBG4S26 / AL-525BW4S26B)	41 mm	21 mm	Alpina's US page prints the precision oddity Lug Width 21.15 mm; the measured tables round to 21mm and the OEM leather runs 21mm at the lugs tapering to an 18mm buckle in 120 x 80mm; buy 21mm [40] [41] [42]
Startimer Pilot Heritage / Pilot Heritage Manufacture (42mm, bumper and true-GMT)	42 mm	23 mm	the rarest width in the catalog: 23mm measured in the owner review, whose author calls it harder to shop than 19 or 21mm, and matched by an owner thread on the same 23 x 18mm OEM taper; almost nothing off the rack stocks 23mm [43] [44]
Startimer Pilot Chronograph (42mm, AL-371 / AL-240 class, quartz and automatic)	42 mm	22 mm	22mm band width on the retail listing for the AL-371 chronographs, and the measured Startimer set's one 22mm row; the even width, unlike the 21.15mm three-handers that share the family name [45] [41]

The 21mm club: Diver Heritage, Date Manufacture, Startimer

Alpina is one of the few brands still building whole model families on 21mm in the 2020s, and the owners feel it. A two-year long-term review of the Seastrong Diver Heritage closes on exactly this: the only complaint is that the 21mm lug width makes finding the perfect replacement strap next to impossible ^[9]. The hands-on reviews print the same figure, 21mm at the lugs with an 18mm buckle ^[6], and note the 50mm lug-to-lug with 21mm between the lugs ^[7]. The practical consequence: 21mm is a thin-selection size at most strap shops, so the choices that exist are worth finding before choosing the watch, or the strap. Alpina's own accessories catalog leans into it, listing leather, rubber, nylon and steel 21mm SKUs alongside the even sizes ^[46], and the secondhand market for OEM 21mm Alpina leather (21mm at the lugs, 18mm at the buckle, 120 x 80mm) is active enough that owners resell them ^[42]. One step further out sits the Startimer Pilot Heritage at 23mm, which its owner-reviewer found even harder to fit than 19 or 21mm, most strap makers stopping at 22 ^[43]; an owner thread documents the same watch's 23 x 18mm factory taper and the custom-strap workaround ^[44]. For how a 21mm strap narrows to its buckle, see the strap taper guide; for why odd widths exist at all, the lug width chart places Alpina's ladder against every other brand's.

The integrated-mount Extreme: where the width is not the half of it

The Extreme case, shared by the Seastrong Diver Extreme and the Alpiner Extreme, is the row where a width number alone misleads. The strap does not sit between two ordinary lugs; its end links are shaped into the cushion case, which is why Alpina's spec block prints two different numbers for one watch: Strap Lock Width 16 for where the strap meets the case, and a 15.25mm strap lock size for the rubber strap itself on the Diver Extreme ^[20], 16 and 16 on the 41mm Alpiner Extreme ^[26] ^[27], and 15.25 and 15.25 on the smaller case ^[31]. The steel-bracelet variants add a third number, a 24.8mm strap lock size for the bracelet version of the Diver Extreme ^[20]. None of these are generic-strap numbers: a standard 20mm or 22mm strap with a straight end will leave gaps at the shaped case. The workable paths are Alpina's own 16mm accessories and replacement rubber ^[46], or strap makers who explicitly list the Diver Extreme ^[5]. The independent corroboration is clean: the stocking dealer measures 16mm lug width on three Diver Extreme references ^[22], the 2025 vintage editions carry the same 39 x 40.5mm case and fitted rubber ^[23], and the collection history explains the shrink from 41mm to 39mm when the Extreme case joined Seastrong ^[24]. The specs aggregator that prints 20mm for the Diver Extreme is simply wrong ^[25], and one large dealer's 18mm for a single Alpiner Extreme reference ^[30] stands against the brand's 16mm field for the same case and that dealer's own 16mm regulator page ^[29]; both divergences are named, neither is charted.

The Carree: 22mm on a 32.5mm case, and a buckle disagreement

The Heritage Alpiner Carree Automatic is the chart's strangest ratio: a rectangular case 32.50mm wide and 39mm long whose strap mounts at 22mm, printed in Alpina's own fields on both the standard ^[36] and 140 Years ^[37] pages, confirmed by the dealer spec sheet for the Edition Europe at 22.00mm lugs ^[38] and the launch coverage ^[39]. A 22mm strap is standard diver territory, which makes aftermarket shopping easy, and the launch review says exactly that. But read the buckle lines before ordering: Alpina's own page prints Buckle Size 22mm for the standard Carree, a no-taper strap, while the dealer spec sheet for the same family prints an 18mm buckle on a 115 x 75mm strap ^[38]. The two buckle figures disagree, so this chart prints both and suggests checking the buckle width of the specific reference before buying a matched set. The strap itself runs ostrich leather, 75 x 115mm in the brand fields ^[37].

Name traps that change the strap

Four Alpina traps do most of the damage. One: Diver 300 versus Diver 300 Heritage, 22mm versus 21mm, one generation and one name apart ^[10] ^[1]. Two: Diver Extreme sounds like the biggest Seastrong and is the smallest, on an integrated 16mm mount no generic strap fills ^[20] ^[24]. Three: the Gyre pair, 44mm men's at 22mm and 36mm Comtesse at 18mm, listed under two different names at the measured tables ^[16] ^[5]. Four: Startimer widths run 21.15mm (brand-printed), 21mm, 22mm and 23mm inside one pilot family, so the model, not the collection, sets the strap ^[40] ^[43] ^[45]. There is also a parts-catalog trap: Alpina's own accessory SKU for the Seastrong Diver Calanda strap carries 21x18 in its part number while the dealer's measured fields for the same strap print a 20mm lug width ^[47], so trust the measured field over the SKU digits. When a listing disagrees with Alpina's own Lug Width or Strap Lock Width field, the listing is the one that is wrong.

Your Alpina is not in the table

Then measure, and treat the case shape as the hint. Pop a spring bar and measure the gap in millimeters as the measuring guide shows; if the strap ends are shaped into the case with no visible gap, it is an integrated mount and the spring bar tool guide plus the brand's own fittings are the path, not a width. The lug width chart places Alpina's 15.25 to 23mm spread against every other brand's ladder, and the band size chart hub carries the full brand index. For length, the second decision after width, the factory rubbers run 80 x 120mm on the Diver Heritage ^[2] and 75 x 115mm on the Extreme and Carree ^[20] ^[37], with the 22mm Seastrong rubber at 105 x 90mm ^[48]; the strap length by wrist guide covers matching those to your wrist.

Related guides

Adjacent references once you have the width: the lug width chart for cross-brand context, the band size chart hub, the strap taper guide for the second number in listings like 21/18, the spring bar size table and tool guide for the hardware end, and the measuring guide for the gap itself. Sibling brand charts: Doxa for the other diver family whose name hides the width, Bell & Ross for the other cushion-case width puzzle, Baume & Mercier for the other odd-width specialist, Grand Seiko, NOMOS, TAG Heuer and Movado.

Sources

Alpina (official US), Diver 300 Heritage AL-525S4H6 product page: Lug Width 21 mm, Band Length 254 mm, rubber strap: us.alpinawatches.com
Alpina (official global), Diver 300 Heritage AL-525S4H6: Strap Lock Width 21, Strap Lock Size 21mm, Buckle Size 18mm, rubber 80 x 120mm: alpinawatches.com
Alpina (official US), Diver 300 Heritage AL-520GR4H6 product page: Case Length 42 mm, Lug Width 21 mm: us.alpinawatches.com
RightTime (dealer), pre-owned AL-520BY4H6 listing: Lug Width 21mm, leather plus additional rubber band: righttime.com
straps01, Alpina Seastrong measured references: 7 refs, Diver Gyre 22mm, Seastrong Automatic 36mm 18mm, Diver Extreme Automatic and GMT 16mm, Diver Heritage Green and Diver 300 Heritage 21mm: straps01.com
Fratello, hands-on review, Alpina Seastrong Diver Heritage: lug width of the case is 21 mm and 18mm on the buckle end, with the odd-width commentary: fratellowatches.com
aBlogtoWatch, hands-on, Alpina Seastrong Diver Heritage: lugs measure 50mm tip to tip, 21mm between: ablogtowatch.com
Worn & Wound, Alpina Seastrong Diver Heritage review: 21mm lug width, 42 x 50mm case: wornandwound.com
WatchUSeek, Alpina long term experience thread: Seastrong Heritage Diver owner at two years, the 21mm lug width makes finding the perfect replacement strap next to impossible: watchuseek.com
Serious Watches (dealer), Alpina Seastrong Diver 300 AL-525LBG4V6: Width 44mm, Lugs 22mm, rubber: seriouswatches.com
Serious Watches (dealer), Alpina Seastrong Diver 300 AL-525LBBRG4V6: Width 44mm, Lugs 22mm: seriouswatches.com
Jewels-Stock (dealer), Seastrong Diver 300 steel bracelet page: Lug width 22mm, 44mm in diameter, 49mm lug to lug, 13mm thick: jewels-stock.com
Two Broke Watch Snobs, Alpina Seastrong Diver 300 review: 44mm case, 49mm lug to lug, 22mm strap: twobrokewatchsnobs.com
Teddy Baldassarre (dealer), Seastrong Diver 300 Titanium AL-525LBBR4V4: Case Size 44MM, Lug Width 22MM: teddybaldassarre.com
Delugs, 7 Best Straps for the Alpina Seastrong Diver 300: prints an 18mm lug width for the 44mm Diver 300, the incumbent error this chart refuses against the 22mm dealer consensus: delugs.com
Mimos Jewelry (dealer), Alpina Seastrong Gyre AL-525LNSB4VG6: Lugs 22mm, lug to lug 50mm, thickness 12.8mm: mimosjewelry.com
Teddy Baldassarre (dealer), Seastrong Diver Gyre Automatic Ocean AL-525LNSB4VG6: Case Size 44MM, Lug Width 22MM: teddybaldassarre.com
Master Horologer, Alpina Seastrong Diver Gyre launch specs: 44mm men's AL-525LBN4VG6 and 36mm ladies AL-525LBN3VG6 Comtesse, rPET NATO straps: masterhorologer.com
Nordstrom Rack, Seastrong Diver Gyre Automatic NATO listing: 44mm case, 12.15mm depth, 22mm band width: nordstromrack.com
Alpina (official), Seastrong Diver Extreme Automatic AL-525K3VE6: case 39 x 40.50mm, Strap Lock Width 16, rubber 75 x 115mm, Strap Lock Size 15.25mm, folding buckle 21mm: alpinawatches.com
Alpina (official), Seastrong Diver Extreme Automatic GMT AL-560LG3VE6: case 39 x 40.50mm, Strap Lock Width 15.25, Strap Lock Size 15.25mm: alpinawatches.com
Windup Watch Shop (dealer), Seastrong Diver Extreme Automatic: Lug Width 16mm on refs AL-525G3VE6B, AL-525N3VE6 and AL-525B03VE6, 39mm x 40.50mm case: windupwatchshop.com
Monochrome, First Look: 2025 vintage-styled Seastrong Diver Extreme editions: 39mm x 40.5mm cushion case, textured rubber with folding clasp, refs AL-525K3VE6 / AL-525B3VE6: monochrome-watches.com
Oracle of Time, The Seastrong Diver Extreme: Seastrong's trend towards smaller timepieces, downsizing from the 41mm case used previously to 39mm, Extreme case with integrated rubber straps: oracleoftime.com
WatchPolice, Alpina brand specs page: Alpiner Date Manufacture 21mm lug; also prints Seastrong Diver Extreme Automatic and GMT at 20mm lug, the aggregator error this chart flags against the brand's 16/15.25mm fields: watchpolice.com
Alpina (official), Alpiner Extreme Automatic AL-525N4AE6: case 41 x 42.50mm, Strap Lock Width 16, rubber 75 x 115mm, Strap Lock Size 16mm: alpinawatches.com
Alpina (official), Alpiner Extreme Automatic AL-525B4AE6: case 41 x 42.50mm, Strap Lock Width 16, Strap Lock Size 16mm: alpinawatches.com
Alpina (official), Alpiner Extreme Chronograph Automatic AL-730NS4AE6B: case 41 x 42.50mm, Strap Lock Width 16, Strap Lock Size 16mm: alpinawatches.com
Teddy Baldassarre (dealer), Alpiner Extreme Automatic Regulator: Case Size 41 mm, Lug Width 16 mm: teddybaldassarre.com
Teddy Baldassarre (dealer), Alpiner Extreme Automatic black dial AL-525B4AE6: prints Lug Width 18 mm, the dealer divergence this chart flags against Alpina's 16mm field for the same reference's case: teddybaldassarre.com
Alpina (official), Alpiner Extreme Automatic AL-525BG3AE6, smaller case: 39 x 40.5mm, Strap Lock Width 15.25, Strap Lock Size 15.25mm: alpinawatches.com
straps01, Alpina Alpiner measured references: 24 refs from 15 to 25mm at the lugs, incl. the 15mm Solarmetre row, 17mm and 22mm the most common: straps01.com
Alpina (official), Alpiner 4 Automatic AL-525NS4AQ6B: case 40mm, Strap Lock Width 20, Strap Lock Size 20mm: alpinawatches.com
Gnomon Watches, 6 Best Alpina Watches: Alpiner 4 Glacier Blue AL-525NS4AQ6B, 40mm diameter, 46.5mm lug to lug: gnomonwatches.com
Alpina (official), Alpiner Date Manufacture: case 41.50mm, Strap Lock Width 21, Buckle Size 18mm, Strap Lock Size 21mm: alpinawatches.com
Alpina (official), Heritage Alpiner Carree Automatic: case 32.50 x 39mm, Strap Lock Width 22, Buckle Size 22mm: alpinawatches.com
Alpina (official), Heritage Carree Automatic 140 Years AL-530BA3C6: case 32.50 x 39mm, Strap Lock Width 22, ostrich leather 75 x 115mm, Strap Lock Size 22mm: alpinawatches.com
Ocarat (dealer), Alpiner Heritage Carree Edition Europe AL-530BBG3C6: Lug Width 22.00mm, buckle width 18.00mm, strap 115/75mm: ocarat.com
Time + Tide, Alpiner Heritage Carree Automatic 140 Years: 22mm lug width makes aftermarket leather straps easy to find: timeandtidewatches.com
Alpina (official US), Startimer Pilot Automatic AL-525BBG4S26 product page: Case Length 41 mm, Lug Width 21.15 mm: us.alpinawatches.com
straps01, Alpina Startimer measured references: 9 refs from 20 to 22mm, 21mm most often, incl. AL-525BBG4S26 and AL-525BW4S26B at 21mm: straps01.com
TimeZone sales corner, OEM Alpina black leather strap 21mm: measures 21mm at the lugs, tapers to 18mm at the buckle, 120mm long end and 80mm short end: forums.timezone.com
Affordable Wrist Time, Alpina Startimer Pilot Heritage review: 42mm case, 23mm lugs, the one gripe is that 23mm is extremely difficult to find strap options for, harder than 19 or 21mm: affordablewristtime.com
WatchUSeek, owner thread: my Alpina has a 23mm lug width, stock strap tapers to 18mm, aftermarket purchases ran 23 to 20mm until a 23-18mm custom: watchuseek.com
Woot, Alpina Startimer Pilot Chronograph Quartz 42mm (AL-371BG4S6B / AL-371NN4S6B): Band Width 22mm: home.woot.com
Alpina (official), Straps & Bracelets collection: 37 SKUs across 16, 20, 21, 22, 23 and 24mm widths in leather, rubber, nylon and steel: alpinawatches.com
Watch.co.uk (official-dealer), Alpina strap ALS-RGRBLK21X18 for Seastrong Diver Calanda AL-525BBG4VR6: strap width 20mm, lug width 20mm, clasp 18mm, 80mm at 12 o'clock and 120mm at 6, despite 21x18 in the SKU name: watch.co.uk
Serious Watches (dealer), Alpina Seastrong orange rubber strap: 22mm at the case, 18mm at the clasp, length 105/90mm: seriouswatches.com