It appears from some comments that the items shown in the bullet list of the final section of this answer bullet list below are probably different from the revisions discussed in the question. On other comment suggestions, I'm leaving that section below to ensure it's made clear.
Hence the likely reason for the missing
C revisions is simply that they were done without a release to the wild, and only the
D revision was considered substantial enough to warrant it.
The business may have already decided that they were going to add
ROR so a released revision before that point would have resulted in wasted effort and possible recall.
Granted, that's supposition on my part but only because we haven't yet found a definitive reason.
Final (deprecated) section here:
For what it's worth, the Wikipedia page states:
- 6502: A 1 MHz chip used in KIM-1 and other single board computers in the mid-1970s.
- 6502A: A 1.5 MHz chip used in Asteroids Deluxe and at 2 MHz, in the BBC Micro
- 6502B: Version of the 6502 capable of running at a maximum speed of 3 MHz instead of 2 MHz. The B was used in the Apple III and, clocked at 1.79 MHz, early Atari 8-bit computers.
- 6502C: The "official" 6502C was a version of the original 6502 able to run at up to 4 MHz. Not to be confused with SALLY, a custom 6502 designed for Atari (and sometimes referred to by them as "6502C") nor with the similarly-named 65C02.
How they relate to your revisions I'm not sure, since the first was the 6502 (not 6502A) and they do not mention a 6502D.