I've seen AA and BA before. On yves saint lauren my AA bottle was different from my AB bottle slightly in terms of modification to the cap and bottle, slightly wider vs taller. I'm certain the AB is newer than the AA bottle just because I bought the AA online and AB from a department store. In your case The numbers must go with the bottle again the higher the number the more recent the bottle. 6AA > 4AA etc.. In terms of the last number its probably the year if that is not possible due to discontinuation then it might be the month although usually its the year.
6 vs 4. Could be the slight modification to their bottles throughout the years. 4 ba -> 5aa -> 5ba -> 6aa etc.
PA vs PC the A probably means aftershave , C for cologne.
I give up.