This is a X-GRIP known as the Mag Spacer, model GL26-27C.
This XGRIP adapts the G19" G23 or G32 compact magazines for use in the G26" G27 or G33 pistols" incorporating the larger magazine into the subcompact grip and increasing the gun's capacity.
Features:
Improved Comfort, Control, and Accuracy
Increased Capacity
Secure Fit