What he said.I'm gonna say December, 1977. The reason is because there is the SUWA logo on the dial. Now your movement (6309) is the same one used in the 6309-70xx dive watch and from 77-82ish (I think) they all had the SUWA mark above 6 o'clock (like yours) which signified they were made at the factory in Suwa, Japan. Seiko then move production to Singapore and the SUWA logo was removed from the dial and caseback. Since all their dive watches were then on made in Singapore (and also Hong Kong) I can't see how a watch made in 1987 would have the SUWA logo. So I will stand on December, 1977 as the production date of your watch.