This is a X-GRIP Magazine Grip Sleeve known as the Mag Spacer, model XGS320/250SCF.
This XGRIP adapts the higher capacity SIG P320/250 Full size magazine for use in the P320/250SC pistols" incorporating the larger magazine into the sub compact pistol's grip and increasing the gun's capacity.
Features:
Improved Comfort, Control, and Accuracy
Increased Capacity
Secure Fit
