The M does not stands for measure! I don't exactly know what the M stands for, but I do know that the first digit stands for the reel number and the last for cue number. So for example when you have 2M6, that means: second reel, 6th cue; 4M2 is 4th reel, second cue, and so on. Alt is alternate end Rev is revised like you said. I think alternate is a different version of a cue (sometimes they record both and choose later. For example: in one version the melody can be played by a trumpet, but in the other version by an oboe). Revised cues are cues that are changed during the composition process after remarks of the director. So they are most likely the cues you hear in the movie. My 2 cents