The first digit of a Seiko serial is the year, the second is the month (1–9 for January to September, then O, N and D). The year digit repeats every decade — a 7 could be 1977 or 1987 — so pick your reference and the decoder narrows it against the documented production window.
The serial dates the case, to the month it was produced, for most mechanical Seikos from the mid-1960s onward. It does not certify that the dial, hands or bezel were born with that case — that is what the authentication files on each reference page are for. A serial whose window disagrees with the reference is itself a finding.