Editorial
The Speedmaster Professional with caliber 3861 is the current-production Moonwatch, the watch flight-qualified by NASA for extravehicular activity in 1965 and continuously produced (with movement updates) ever since. The 3861 introduced co-axial escapement, a Si14 silicon balance spring, and METAS Master Chronometer certification in 2021; the silhouette is otherwise unchanged.
The Speedmaster Professional traces back to 1957 (CK 2915). The 3861 replaced the caliber 1861 in 2021 and is the first co-axial Master Chronometer Moonwatch, the movement is METAS-certified to ±0/+5 seconds-per-day accuracy and resists magnetic fields up to 15,000 gauss. Cosmetic changes are subtle: a stepped dial, applied "Omega" logo (vs printed), and a slimmer bezel ring.
The hesalite-crystal variant retains the period-correct look; the sapphire-sandwich variant trades closer-to-original look for clarity.
Common things to check: hesalite vs sapphire (price, weight, and resale market both vary); bracelet (the brushed-link Speedmaster bracelet ships with the watch but many sellers separate it, confirm it is included); dial freshness (creamy-aged tritium lume from the older 1861 sometimes shows up in pre-3861 watches misadvertised as 3861); case-back engraving (the 3861 case-back shows the seahorse and the Master Chronometer wording).
