The Magpul Gen M2 dust covers work on Gen M3 magazines.
But not vice-versa.
Since you can buy three packs of the Gen M2 covers, that could be handy to cover losses of the easily lost things.
The reason the new covers don't work on older magazines is the two lugs pictured here (M3 in purple):
Of course, without those lugs, there's a bit of a gap at the back of the mouth: