On Mon, 28 Sep 2015 08:11:16 -0400, Keyser Söze
wrote:

The 10-rounders, I am told, are simply 20-rounders with a "stop" welded
inside so they cannot be converted to 20-rounders, and that makes the
10-rounders legal for sale in states where larger cap mags are
restricted (such as Maryland).

There are several manufacturers of true 10 round mags.
Those you are talking about are simply "loophole" products that are
pretty easy to convert to 20s.
Typically they just weld an extension on the follower that will break
right off (they did on my M1A mags), or you can get a new follower.