Omega Seamaster 300 Steel, black dial, 1957–1961

The original Seamaster 300. Launched in 1957 alongside the Speedmaster and Railmaster as Omega's 'Trilogy', the CK2913 established the diver Omega has returned to ever since: a roughly 39mm steel case, a rotating bezel with a Bakelite insert, no date, and an automatic caliber 501 (US-market examples carry the lower-jewel caliber 500). Specialist references track several sub-references, CK2913-1 through -8, whose bezels and handsets vary; the broad-arrow-and-lollipop configuration seen on examples such as the CK2913-7 is not universal across the reference.