It's quite easy
Mark a line EXACTLY
1 11/16" from the TOP of the mag. This is the TOP
edge of your magcatch notch.
*If uncomfortable with your measuring abilities, mark it slightly lower. You can tweak it later (like I did on the left one in the pic - fixed now)*
Use a Dremel tool with a side-cutting bit - flat tip, and carefully cut a notch into the mag. Don't go too deep and cut through! **UPDATE** Cut the notch very shallow. On one of my mags, the magcatch dented in the thin plastic. This would slightly bind the BB follower and cause misfeeds.
Hopefully, you get something like so....
Test fit it in the USP to make sure the catch works. If it doesn't catch, then your notch probably needs to be extended upward a tiny bit.
If you try to fire it, you'll notice that the trigger doesn't return forward.
The trigger bar rubs slightly on the TM mag binding the mech.
Another easy fix!
Just file or sand down the area(s) circled in red. This will cure the binding problem.
And that is it!
A bit weird looking, but sexy!
The modified mags will still function flawlessly in a Glock AEP.