Strap for Speedmaster Professional Moonwatch Sapphire Sandwich / Strap

The lug width of the reference 3873.50.31 is 20mm. This measurement is specific to this model and should not be confused with other dimensions. To verify compatibility, measure the distance between the spring bars that hold the strap at the case ends: they must be exactly 20mm. A strap of different size will not fit correctly and will compromise both the aesthetics and functionality of the clasp.

The Moonwatch pairs beautifully with alligator or shell cordovan straps in vegetable tanning, especially in dark colorways such as black or tobacco brown. These materials reflect the historical instrumental aesthetic of the watch. For those seeking practicality and visual versatility, heavy cotton NATO straps offer continuity with vintage models. FKM rubber remains the pure technical choice for extreme environments—the one the Moonwatch was certified with for the Apollo missions.

To change the strap on the 3873.50.31, you need a spring bar tool. Position the watch with the lug side facing upward, insert the tool into the spring bar slot to compress it slightly, then extract the bar. Repeat for the second spring bar and the strap will be released. Proceed in reverse to install the new strap, ensuring that the spring bars fit perfectly and that the strap is symmetrical with respect to the case.

Yes, provided it is made for 20mm lugs. Quality artisanal aftermarket straps, such as those offered by MilanoStraps, are fully compatible with the Moonwatch. In fact, many collectors prefer custom-made vegetable-tanned straps because they offer superior workmanship and customizations impossible with original straps. The important thing is to choose manufacturers that respect millimeter tolerances and use excellent materials.

The lifespan of a leather strap depends on the material and use. Vegetable-tanned alligator, if well maintained and not exposed to constant moisture, typically lasts 4-6 years of regular wear. Shell cordovan, thanks to its more compact fibrous structure, can last 7-8 years or longer. FKM rubber straps have similar longevity (5-7 years) but are more resistant to moisture and extreme environments. Regular maintenance—avoiding prolonged water exposure, protecting from excessive sweat, occasionally applying specific oils—significantly extends the strap's life.