Diopters from a Pentax Spotmatic/K body, Contax/Yashica RTS FR etc, Ricoh will fit. These use an eyecup to hold the round drop in diopter. Olympus OM round diopters should fit (but the Olympus eyecup will not).

I think early Canon & Minoltas use the same eyecup fitting.

I use the same +2 diopter in an eyecup on my M645 prism, Spotmatic SL, and Yashica FX-D cameras.