Thanks guys, very solid and aluminum. Basically a 3 slot 1/2" QD riser from UTG along with the UTG folding mechanism I use for the Tapco stocks. I used a self etching primer then matte black fusion paint to refinish it. It was a one-off prototype per request so the guy gets what he gets. I was going to send it out in the raw but then remembered I had the primer and paint.
FYI, the MPX is a 9mm pistol/SBR version and the MCX can be either a pistol, carbine, or SBR in 5.56NATO, 300BLKOUT, or 7.62x39. Loosely built on the AR platform.
I ended up on the SIG website to get all that info. High $$$ item, $1k-$2k depending on options.