This is a X-GRIP known as the Mag Spacer, model HKUSPC-P30.
This XGRIP adapts the HK P30 high capacity magazine for use in the USP Compact" incorporating the larger magazine into the USPC grip and increasing the gun's capacity to 15 rounds in 9mm and 13 rounds in 40 cal
Features:
Improved Comfort, Control, and Accuracy
Increased Capacity
Secure Fit
