Well, ya'll are forgetting that Frankie betrayed Mike because he thought Mike tried to murder him. In the same situation, Clemenza might have betrayed him also. But between them, it was only Clemenza that we know had sworn fealty to Michael(final scene of I). So, without his attempted murder by Michael, I find no basis for Clemenza's betrayal.
Also, I would think that it is typical of movie contracts that representations of players can be used in sequels. Afterall, Clemenza is referenced in at least two scenes. So, I don't find arguments about using his and Vito's pictures convincing.
There are certainly situations below attempted murder that could have caused Clemenza to turn. Perhaps something as simple as non-support while his territories were chiseled away, combined with a deaf ear to his complaints.
As to the contracts, I don't know what a standard SAG contract looks like but I frequently am involved in contract negotiaions with other types of performers and I have some knowledge of contracts involving SAG's partner union AFTRA. Based on that, I find it highly unlikely that a standard actor's contract would include a clause as you describe. Since sequels were far less prevalent in the 1970s, I find it even less likely that it would have beeen standard then.
