As I've figured out both first hand and after the fact through the knowledge base on this website, U-notch don't play nice with most extended base plates, some of which will pop off under load tension alone. When they bulge in the middle, they restrict by the floor plate allowing it to squeeze out of the grip of the extender.
And that's not to mention that the old (maybe first gen?) U-notch aren't even long enough to use in modern Glocks, P80's, or PCC's--another hard lesson I've learned first hand. They just barely click in (if at all), and even if you can get them to load in the first shot, will often cause later FTF or premature slide lock. If you do a side by side comparison with modern gen Glock mags, you'll see they are like 3-4mm too short. I have to agree with Drgrant--Unotch are garbage. That's not to say $180/square is right though.