No, they sent a replacement follower as well, and that's yet another piece, and also a replacement spring. I guess I didn't think about that as being part of the extension.
I'm not exactly sure how to explain it, especially since I'm not quite sure WHY it's a separate piece. Maybe if I write out the steps to put it together...
First I took off the old cap and weaseled out the "ring" that held the spring in, and took out the spring. I replaced the old spring and follower with the new one and dropped the spring in. Now I've got the cylinder + collar, and the mystery part. Now, if I were to put on the tube right now without the mystery part, the front of the magazine + extension would actually have a hole in it. To plug it, I drop the mystery part down into the cylinder before putting it on. Then I screw on the cylinder.
If I press on the mystery part, it compresses the spring, but I can't find that this actually does anything. Furthermore, in my limited experience, no other extension I've ever seen has this as a separate part. Like I said, I don't really get why it is.