This listing is for a slightly used Matador Arms Mag-X Magazine Adapter for Sig P320 magazines. 
 
From OEM:
The Mag-X P320 allows you to use your pistol magazines in any AR-15 milspec lower – the ultimate PCC-pistol combination for Sig®-P320.
 
Fits any standard AR-15 lower receiver.
No modifications required to your lower or magazine.
Installs in seconds.
Built-in magazine release.
Locks into place with the standard AR-15 catch.
Lightweight aircraft-grade aluminum construction; anodized finish.
Steel ejector and magazine catch.
 
*Only compatible with mil-spec AR-15 lower receivers, combined with a 9mm upper with a "Glock-cut" bolt.
*No last round bolt hold open.
*Hand-fitting may be required with FM-9 uppers.
*The MagX will not work with the CMMG Radial Delayed Blowback.