This is a Pearce Grip known as the Grip, model PG39-SUB.
These units replace the magazine base plate and internal floor plate. They will add three rounds to the Glock M26" two rounds to the Glock M27 and M33 and one round to the Glock M39. These extensions will add 1" additional length for better control and comfort.
Features:
High Impact Polymer
Increased Comfort and Control
1 Additional Gripping Surface