Okay, the Model 7700 was produced from 1970 through 1978. Some specs evade me, such as engines. I know the first few years saw gasoline powered engines, but at least by 1975, the 7700 was diesel-only. Diesel or gas were opted on the other models for most of their history.
Cabs changed a little in 1973 or 1974. A long extension or visor was added to the front of the cab that year. That visor only lasted 2 years. Other than that, the cabs were the same style. The seats were all yellow vinyl. The brown fabric seats did not come out until later on with the Titans.
The front engine cover of the 7700 was changed in 1974 from the vertical ribbed grille to the more stylish solid cover, with the ventilation at the bottom of the engine deck. Also, the new rotary screen was added. These were really very pretty combines in their day, too. In 1975, a guard rail across front of engine deck was mounted. That first year, the platform to stand on, was the fold-down bottom "lip" of engine compartment. By 1976, there was a regular catwalk in place, not related to the lower portion of engine cover.
I know of more improvements and minor style changes, but just can't think of all of them and the years each came out, but overall, the John Deere 7700 cut, threshed and separated its way right into my own heart, making it one of my all-time favorites, a classic of the 1970's, and a combine worthy to this day of protecting and preserving, should a fine one be found.