1981 Skilsaw 77 with Original Box

http://www.ebay.com/itm/VERY-RARE-S...848?pt=LH_DefaultDomain_0&hash=item19db81d6d8

How do I know this model 77 is from 1981? The serial #, SHS4, breaks down as:
S = Skokie, IL assembly plant
H = calendar year 1981
S = calendar month May
4 = production shift

Also... a well-worn c. 1978 model 867 Super Duty 7-1/4" circular saw.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=330910978743

~Ben