https://wornandwound.com/affordable-classic-omega-speedmaster-automatic-reduced-ref-3510-50/

The Omega Speedmaster Automatic Ref. 3510.50 is commonly known as the “Reduced” because of its dimensions. A 38.5mm diameter is slightly on the smaller side of average for a modern watch, and a chronograph at that, though it’s nothing extreme. The lug-to-lug distance comes in at only 45mm, and this is the most significant figure in how the watch wears on the wrist. Additionally, the measurement from the case back to the top of the domed hesalite crystal is just under 12mm, which is impressive for an automatic chronograph. The snap-on case back features Omega’s hippocampus engraving. Water resistance is rated to 30m, which is firmly in the “washing hands” territory.