I've been doing some research on this as well, but am not done. Here is what I've got so far:
The first picture is from the Jan 67 MPC. I believe the MPC to be wrong because there is no differentiation between PS and MS. It is interesting to note that we are dealing with only C7 numbers, not C4.
The second picture is from the 65-72 MPC. Here the 5/1/67 date is introduced, and whereas the 67 MPC referenced a -A and -B C7 part, here a -C part is listed. Also here is looks like the -B is clarified as MS, and -C is PS. Again these are "late", and the C4 is early.
It is very interesting that the 65-72 MPC from 75 does not even list the C9 part, which one would expect. Strange.
It will be a couple weeks before I can check my collection. I do have at least one of each C4, C7, and C9 PS arms and need to measure them. I have one or two short shaft 1" boxes, with original arms.