It's been a while now but I remember a thread about someone having to file/sand the very small stair step off the 45c release in order to get it to latch into place correctly on a USP with polymer magazines.. Unsure Why?
I'll put up a picture of both & you can see it..
The arrow shows the step that was removed in order to get it to fit correctly.. It's the stop where it rests at the bottom of the frame..
Almost looks like the USP Release has been filed on in the lower photo.. You can see the strokes in two directions.. LOOK..
Wish I could find that post, Probably @ HkPro?? Unsure Now.. My Mind has been a mess.. Forgive Me..
I'll Keep Searching & Should Find It..
This is a modification that I want to try on both my USP 40 Compacts & will get a couple of them soon enough..
Hk 45c Magazine Release
Hk USP Magazine Release