#5,7, and 8 followers
As noted above, you either have a drop free mag or not.
However, some of my observations regarding G22/23/27 models (.40 cal) are that some of the magazine followers are stamped with the part number 5, 7, or 8. The part number is right on top of the follower when looking at an empty mag.
The one with the #5 follower is not a drop free mag, while the ones with #7 & 8 are drop free. All three followers are different in shape from each other, so I can arguably say that there are at least 3 generations of magazine for .40 cal Glocks.