I don't know why I was so dense, but in a moment of clarity last night (doesn't happen often), I figured it out. It's completely obvious, it's the first letter of the color(s) for the wire that attaches to that terminal, in Italian of course.
For the tachometer:
BN = Bianco/Nero (white/black)
N = Nero (black)
AN = Azzurro/Nero (light blue/black)
GN = Giallo/Nero (yellow/black)
M = Marrone (brown)
And so on. Well, duh...

-Bryan