Ah, the mysterious 1m1!
Well, actually there’s a simple explanation:
When writing music for film, a numbering system is used to keep track of the various musical pieces. The first digit stands for the film reel number (from back when films were edited in reels), the letter “m” stands for “music” (makes sense!), and the last digit stands for the position of the musical piece in the reel. So, “1m1” is the first musical piece in Reel 1; 1m2 would be the second piece in Reel 1; 2m3 would be the third piece in Reel 2, and so on…
Oh, and since 1m1 is the first cue in Reel 1, it also oftentimes refers to the Opening Credit music.
Simple. Mystery solved!