Hard to describe without pictures. I'll post some when I get home unless Nick gets to it before me. The M90 magazine is much simpler. Instead of having the standards M95 follower arm, the M90 is a single curved arm (similar in concept to that found on the Dutch M95). The M90 bolt is also a little different - the cocking piece has a round, knob shaped end, unlike the M95. Also, the rear of the M90 bolt is taller and more massive than found on the M95. Again, hard to really describe without pictures. There were lots of comparisons by both Nick and myself on the old forum.