What is different between the MCX and MPX brace that makes them not interchangeable? — 2 Answers

I can attest that this will work with both the MCX and MPX - but specifically on on an MPX Gen 3 (M-lok with upgraded gas block). I can't speak for earlier versions.

No question that the fitment on the MCX is perfect: slide-through and mounting are flush, and the top of the rail mount lines up perfectly with the end-taper on the top of the Pic-rail for the MCX. For the MPX, not so much - part of the receiver's pic-rail will show, but it will mount and slide in perfectly without issue. THE ONLY CAVEAT is that once you torque the screw down, you'll find that it will thread into the rail, as it seems the guides on the MPX and MCX rails are slightly different. But --- no mallet nor press required! Just... don't be upset if your pic-rail doesn't look "mint" anymore. (Not that anyone can really see this.) Also, if you're one who depends on the MPX mounting holes for the side-mounted QD piece, the pic-rail mount on this brace will cover up those mounting holes. Not a huge loss, since the QD mount position on the brace is infinitely worlds better from a placement perspective anyway. (At least, IMO, and especially for single-point sling usage.)

Hope this helps. Still easily the best, sturdiest, and sharpest-looking collapsible brace/stock system for the MPX/MCX. (And I have three of these, FWIW.)