Seiko serial numbers are typically found engraved on the case back of the watch and often follow a six-digit format on vintage models. The first digit commonly indicates the production year, while the second digit is often associated with the production month (where 1-9 represent January through September, and O, N, D are sometimes used for October, November, and December). However, this system can vary by production era, and some references suggest that the month coding changed over different decades.
Because the year digit is a single number, the serial number alone typically cannot distinguish between decades. For example, a serial beginning with '8' could potentially indicate 1968, 1978, 1988, or 1998. Collectors often cross-reference the serial number with the movement caliber, case style, and dial characteristics to narrow down the production decade. It is worth noting that Grand Seiko models may follow a different serial number convention, particularly newer references produced after the brand's relaunch as a separate entity.
Some modern Seiko watches, especially those produced for international markets, may use longer serial number formats or different coding systems. Always consider that serial number formats can vary by factory, production line, and era. When in doubt, consulting Seiko's official service centers or reputable collector communities is commonly recommended for accurate dating.