As you can see the notch on the top of the spine is slightly narrower on some of the XCR-M pistol magazines. It is the one on the right. Some fit fine, some don't. You'll need to open the notch up a bit with a file...on the left side looking at it, for the bolt to release on an open bolt. There seems to be some slight variation on the mags.
I'll post another mag shortly.