This is a Pearce Grip Magazine Base Plate known as the Grip Ext, model PG-1045+.
This extension replaces the magazine floor plate adding two rounds capacity to Glock factory M20"M21"M29"M40 and M41 magazines. This unit is for use on high capacity magazines only" it will not work on the 10 round M20 and M21 magazines. Not for use on Glock M30 10 round magazines.
Features:
High Impact Polymer
Increased Comfort and Control
Fits Gen3 and Gen4 Models
