Unless the head badge is a replacement, I would bet on that date code first. Now you never know, Schwinn may have had the frame sitting around for a while before they did final construction.

Google Schwinn Chicago factory, exact closure date is there.

Check your bike specs versus the 1982 and 1980 catalog pages:

http://www.trfindley.com/flschwinn_1...0/1982_11.html

http://www.trfindley.com/flschwinn_1...0/1980_06.html

Note, September 1980 frame puts it likely into the 1981 category as a minimum, and makes the 1982 headbadge pretty believable.

1982 had Shimano Altus derailleurs, 1980 had Shimano 400 derailleurs.