How sure is any of this?

Not affiliated with Seiko. Seiko Almanac and Tiny Hour Tales are independent. “Seiko”, “Prospex”, “Presage”, “Grand Seiko” and related names are trademarks of Seiko Watch Corporation, used here only to describe and identify watches.

The serial scheme is a convention, not a standard. Seiko has never published an official method for dating watches by serial number. The decoder uses the widely-accepted community reading: it works for most Seiko from 1966 onward, but can be wrong for Grand Seiko, some quartz lines, and any watch whose movement has been replaced during a service. Treat results as strong estimates.

Caliber dates are approximate. Production windows are drawn from community references and are marked with a “~” where sources disagree. They’re good enough to narrow a decade — not to certify a single year.

If you spot an error, we genuinely want to know.