This is a X-GRIP Magazine Grip Sleeve known as the Mag Spacer, model XGGL43-9.
The XGGL43-9 XGRIP adapts the ETS 9 round 9MM magazines for use in the G43 pistols" incorporating the larger magazine into the grip and increasing the gun's capacity.
Features:
Improved Comfort, Control, and Accuracy
Increased Capacity
Secure Fit