well, I am definitely of the opinion that you should NOT date a Fender Bassman until its is at least 16 or 17, perhaps even 18 in some states.
... sorry could not resist
good snag. I have a '78 Fender Studio Bass that just kills and I was able to pin down its manufacture date by the codes on the Transformer. A kind and knowledgeable TBer helped with that.
someone will be along shortly to help you with a real answer.