Gary : Before getting too deep in your discussion, there is a semantic issue you need to address. In John's excellent explanation of suffix letters, you apparently read his wording incorrectly. In his discussion, he talked about "revisions", or changes, not "reversions", or going back to something previous as you have noted.
That being "out of the way" now, I, like Jim, have a question as to what your source was for the identification numbers. I have looked at the "1965 only" MPC, the "1960-68" MPC, and the "1968-72" MPC, and found the following "C5ZF"carburetor identification numbers specified for 1965 A code Mustangs ;
1) 1965 Only - C5ZF-C & D
2) 1960-68 - C5ZF-C, D, E, F, J, K, L, & M
3) 1965-72 - C5ZF-C, D, E, F, J, K, L, & M.
Since the 1965 Only MPC was printed January 1965, it is possible, and in this case highly probable, that additional revisions took place prior to your build date, Unfortunately, it is not known exactly when the "E, F, J, K, L, & M" revisions actually occurred. This is why it is always bes5t to use a source closest to your build date, and that includes the time frame when your car was built. In this specific case, it appears that no additional revisions to the carburetors were made btween the printing of the 1960-68 MPC (1968), and the printing of the 1965-91 MPC (May 1975), but this is not usually the case.
Bob