I took at look, and for future searches note that we are talking about basic part 7A572, "Bracket Assembly - Clutch Release Equalizer Engine" based on the picture in the first post.
The first two pictures from the 1968 version of the comprehensive 1960-1968 MPC. I don't see any note of a running change here.
The 65-72 MPC circa 1975 lists a revision B of the part, but since the -A from the 1968 MPC is not listed, it should be considered interchangeable.
I suppose that we are discussing a -A vs. -B change, although I don't know that for a fact. However, the evidence sure points in this direction. It is interesting that some revisions will retain the prefix (C7ZZ in this case), while other times they make the change in year (to C8). I've seen both ways and have not come to any suspicions or conclusions, but I digress.
Someone with OSI books could shed some light on when rev A was obsoleted. That will get us closer (considering 1968 vs. 1975), but still not when the change was made at the assembly plant. Sounds like Bob has seen enough of them to provide a plus or minus date, so that is exactly the type of real world feedback needed in a case like this.
