1976 says my SCSW 2nd Edition.
That date is merely a known "range" and doesn't really accurately indicate either a ship date or build date... but it does give a clue as to when the frame was made and stamped with a serial number. Smith & Wesson is absolutely famous for jumping around quite a bit with parts and frames and such.
A factory letter would tell you specifically when it was shipped out and where they shipped it, and it would tell you what features it had when shipped.
