Difference between H11L1 and H11L1M
The H11L1 was originally made by General Electric, but cloned by many other manufacturers. Almost everybody also calls it "H11L1" (clones with different names, like the PC900, turned out to have not been as successful).

Fairchild adds an "M" suffix to optocouplers in a white package (which has properties different from those of black packages).

For MIDI, the differences between the clones do not matter. (The PC900 had looser tolerances than the original.)
For your purposes, it's actually the same chip.

(As it happens, General Electric's optoelectronics group, after being acquired by Harris and then QTC, ended up at Fairchild. So, technically, H11L1M is the official name nowadays.)