Shadow Panther is the one strange exception to what I was saying, given that if you were right, Cordon should be MP-39P rather than MP-42. I would agree that exclusives will automatically get a separate letter, while the + versions will either be cartoon or (as of now) toy accurate to plug a gap in the collections of one group, the other or completists.
To clarify, I was referring to mainline releases. After all there is one Masterpiece what isn't on that list, because it wasn't a special edition, which had a letter designation - that's Soundblaster, which had the MP-13B designation. If every single figure which could be classed as a variant were still getting variant lettering, then Delta Magnus would have been a MP-22 (possible either R or D) variant rather than MP-31 and Cordon would be MP-39P rather than MP-42.
What this suggests is that we're both right here. What I was saying about main line releases was spot on - anything main line will have individual numbering from now on without variant lettering, even if it is a clear variant, like Delta Magnus or Cordon.
What you were saying about every single non-mainstream release, that it will always have a letter designation typing it back to the original mould, is also correct.
The one exception here are the + editions, which regardless of how they are released, will always refer to either a cartoon or toy accurate release, depending on whether the original release was more toy accurate or cartoon accurate.