If you take a close look at the bottom portion of the back panel of a .40 Glock mag you may notice a 1 or 2 to the right of the logo. Wtf does that mean anyway? This has been on my mind for awhile and guess what happened next? Research ensued.
I did find some useful knowledge out- thanks to voyager4520 on glock forum.com:
It signifies one of the iterations of magazine tube design. The early “ambi-cut” .40 mags had no number. Then there was a tube with a “1” that came with #8 followers, then there was one with a “2” that early on had #8 followers and now has #9 followers.
Got all that? The reason you should care has to do with mag repairs or upgrades-the reason those followers have different part numbers is that they aren’t identical. If you replace the follower in a #1 mag or an early #2 mag with a #9 follower the slide stop won’t fully engage on an empty mag- the #9 follower is just a little too wide for those mag tubes.
You can correct this by removing material from the top of the tube where the wider body of the follower is rubbing. It may also help to remove material from the top edge of the follower- you will see the rub mark develop and here is a great photo showing the spot:
That’s all good info- so to bring this all together here is a great cutaway image showing the magazine follower in action. (Hint: the mag follower is the black piece you see under the lowest round in the mag):