According to the Sunfish Age and Hull ID article in the database, "The cockpit was redesigned for the 1971 models, which started shipping after the end of August shutdown 1970." In the model year format pre-1984, the months are coded as A=August through L=July. So if they did an August shutdown every year and used that as a cutoff to change model years, it would follow that the 1975 models were manufactured from August 74 through July 75.
Based on having seen a whole ton of boats over the years, I don't think an early shutdown (like July) was the norm, but who knows? In any event, that boat is a 1975 model year, and as the boat is 35+ years old, its specific year of manufacture is probably not too relevant. If anyone really wanted to know, Howie at http://cctexan.tripod.com/amfboats/index.html used to work at the plant - he may recall when they usually switched over. BB
I looked in the index but couldn't find a specific way to date my Serial number, # 30971. I think it was originally bought circa 1965. Still sails great and am enjoying posts on this site to install a few improvements, eg a stand-up ratcheting mainsheet block and maybe a new mast cap with the more modern halyard fitting.