Kartwheel68
Husqvarna
Pro Class
Again I'm going to state you guys know 1,000 times more than me but I do know manufacturing. If by the end of the run of 76's isnt it possible that the coming 77 parts like the air box and mag cover would be used on any remaining 76's vs making more of the 76 parts for a couple hundred bikes? This bike just doesnt have the feel of one cobbled together.
With Husqvarna the opposite, that leftover '76 parts got used on early '77s before the newer parts were in, is infinitely more likely. In addition, the WRs would have been the last bikes to get "new" parts, they would have gone to the CRs first, WRs always got stuck with leftover stuff.
Having said that, your bike does not look cobbled together, other than the shocks, airbox cover and the mag cover, its pretty stock and those things would have been common owner swaps at that time.