Kinda goes back to Stan's original idea and makes a kind of ironic twist.

Good idea, things better left unseen, better left to the imagination. And something Marvel should learn. Marvel would not miss the oportunity for making this storyline a big crossover event and they would have a whole side miniseries devoted to tell that fight (even worse they would have one for the Avengers, another for the FF, and another for the origin of Magneto´s team...  The more milking the better for them). 

Maybe the ‘M’ would be the same concept. Rather than standing for ‘Magneto’s Men’ it would stand for something important and superior. 

The Magnificoes. 

The Magnificent Men (Mag Men for short). 

Or Mutant Militia (M-Militia).

Or as someone else said, simply ‘Mutants’ and ‘Mutts’ for short. 

Maybe Magneto would name them something humiliating like ‘Magneto’s Maggots’ 

However, Magneto is probably so self-obsessed that the M does indeed represent his ‘brand’. Ownership. ‘Magneto’s Men’ again, Mag-Men for short. 

Right, back into my hole then. :)

;)

How meta would that be?

*cue head exploding