It looks as though there is just a tail of plastic that stoppes it at 10 rounds. Should be easy to dismantle mag and remove the extra. Will update as to reliability when they arrive.
Almost forgot to thank the OP for posting the link!!
make sure you give a range report on how the Promags work. I have had nothing but trouble with their pistol mags and will never buy another. it may be different with the 15-22 mags, but as for handguns, forget it!
If you are referring to the promag 10 round, someone on this forum explained how to do it. I think he had to cut the follower since it's molded in one piece. It does not use a pin like the S&W.
Anyone have a link to this post? I can't seem to find it. My 10 rounders arrive Thursday and trying to figure out how to do it beforehand.