76 & 77 are virtually identical, usually the 77's have the 2 position foot pegs and came with Marzocchis but i have heard of Cerriannis on the 2 peg models and mazocchis on the single position peg models
I personally would think a motor produced in August of 76 would of been in a 77 model, but that's just my opinion,frame serial numbers had no break point between 76 & 77 just a continuous run of the 550 numbers.
The frames were different where seat mounted.
Here is a pic of a 77 frame