Others are more knowledgeable than I, but these are some differences:
- fuel tank
- seat
- carbs(slightly leaner jetting, IIRC)
- exhaust(as mentioned, 4-1 vs 4-4)
- frame(small differences to accommodate 4-1 exhaust)
- side covers
- an "F" in the serial vs "K"
- perhaps the front fender with the single brace is F specific(?)
Ken
(1975 CB550F)