I got two in the mail today...my intention was to install them on my 12 rnd .40 mags. Alas, I ran into what sounds like the same issue as you described. However, I don't seem to be able to use the stock base plate like you said. If I do, the extention does not stay locked into the mag, and could slide off quite easily it seems. I took apart a 10 rnd mag, and the spring and follower are definately different. I swapped the follower and spring between the two mags, and now everything works. I'm wondering about reliability though....I haven't be able to fire them yet, though I've loaded and unloaded them both several times and everything seems ok....any ideas? Am I cooking up trouble?