More plastic and less metal.
Also, as I remember, the caps are all 1-piece and have row contours for varying heights like regular keyboards. The spring/pivot plates are probably the same, and regular M stems will fit the barrels and operate properly, but I think that they are mostly (way) too tall.
This is from memory, I have not used M2s very much, have never opened one up, and have not owned one in a couple of years.