It’s factory sig and if excellent quality. - SIG SAUER Stock Adapter - 1913 Interface Folding Knuckle

This is a good choice for MPX owners and any similar weapon that uses the rear picatinny stock mount. The buffer tube screws in and is held in place by a set screw. This saves the frustration of using a castle nut in this application. The set screw engages the slot in the buffer tube and gives a wobble free interface. This will also fold. It takes a little force to get it to release so you can fold it. The benefit is, it locks up solid when deployed. It’s a very tight and completely wobble free engagement. The feel will be familiar to anybody who’s spent time with an AR platform rifle. The alignment is perfect for most optics and the option to extend the stock (beyond the length of the “2 rail” style stock or brace) is a really nice benefit. It makes the package feel more natural to me.