The one on the left has the "bolt hold open" feature to show when the last round has been fired (by holding the bolt open), the one on the right does not.

Was the last round hold open mag an M2 thing, or were they used on earlier mags as well?
The only hold open one I have is a 5 round one.

Billy, approval was granted on Oct 26 1944 for the development of the improved (Hold bolt open) follower, named the T18. So it is fair to say that early and mid production 15's had the old follower but later ones had the T18. All 30's had T18 followers. You gotta keep an open mind however as it is possible to place a late follower in an early mag and vice versa. Mike.