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.
I'm not sure but I think the "no number" tubes were the same design from #6 followers up to #8.
The "no number" tubes had much slimmer protrusions of plastic at the front of the feed lips that reached further down into the tube, and the inside of the tube was a little wider in certain spots. The "1" tube had larger plastic protrusions that reached shallower into the tube, I'm not aware of any other differences. The early "2" tubes had the same plastic protrusions at the front of the feed lips as "1" tubes, but the dimensions inside the tube were a little smaller. Then they received #9 followers, but in some of them the #9 follower would fail to rise fully because the part of the #9 follower that engages the slide stop is wider. So then they cut away some plastic at the top of the tube and slightly widened that part of the metal liner to allow the #9 follower to rise fully. Now the latest "2" has the plastic protrusions shaved down after they're made. The 22-round .40 magazines are essentially "2" magazines and they progressed through all of the same changes that the "2" magazines did.
#9 followers are supposed to work in .40 magazines that came with followers #6 through #8, but I've found that because the part of the #9 follower that engages the slide stop is wider, sometimes it may not rise fully and you may have to shave some plastic out at the top of the tube where it rubs, and you may even have to shave some material off of the follower where a rub mark will appear. Here's a picture of the part of the #9 follower that is wider, may develop a rub mark, and may have to be shaved down to work in a tube that came with followers #6 through #8:
I had a G23 "1" and "2" magazine that were brand new cause failures to feed while the rest of my magazines worked fine. They both had the wider plastic protrusions at the front of the feed lips, and after shaving them down the magazines worked flawlessly. Apparently Glock's engineers are now aware of the problem because the latest "2" magazines have those protrusions shaved down after they're made.
The same problem was reported in G22, 22-round .40, and G32 magazines. The same protrusions are present in every .40 magazine, though I've never experienced or heard of it being a problem in G27 magazines.
Wow!!! Thank you for the info voyager. You're information made me glad I asked the question.
Now. The follow up question... the only manufacturer to do this and is there any advantage or disadvantage to having any particular number on e mag. Being that it affected a older part in the gun I'm guessing no but I'm a curious Jew.
No advantage or disadvantage. If you've never had any problems with that magazine there's nothing to worry about.
If you ever decide to replace the #8 followers with #9, you may have to do some light shaving to make them work properly. I have plenty of magazines with #8 followers and they're not inferior to #9's in any way, in fact I'd prefer to keep the #8 followers in them.