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: