the f2 also keys cassettes -- that's why you have to turn that little handle thingy on the bottom to open the back. I sincerely doubt Nikon would have made a different cassette --- backwards compatibility was more popular back then.

Leica did a similar thing -- black knob Leica film cassettes will only work in LTM cameras, but those with chrome knobs were made so they work in both LTM and M.