The following is from the Standard Catalog of Smith & Wesson, 2nd Ed. by Jim Supica and Richard Nahas:
A475001_____ 1978
A5824XX_____ 1979
A5841XX_____ 1979
A7175XX_____ 1981
A7981XX_____ 1982
Looks like 1980 to me but you'll need a factory letter to know for sure.