Omega serials live on the movement for vintage watches, and here is the trap almost every lookup site falls into: the Speedmaster has always used its own serial sequence, separate from the rest of Omega's production. Date one on the standard Omega chart and the answer can be wrong by several years. This tool uses the dedicated Speedmaster chart, cross-checked against each reference's documented production window.
The serial dates the MOVEMENT, on the Speedmaster's own sequence, to an estimated year — and movements sometimes waited a year or two in inventory before being cased, so a watch sold slightly later than its serial is normal. Charts are collector-compiled and approximate. After 1999 the Speedmaster moved to a new numbering system, and around 2019 Omega randomized serials entirely; for those, the warranty card is the record. A serial whose era disagrees with the reference is itself a finding.