This unit installs in the bottom rear cavity of the frame/grip, utilizing the lanyard hole for retention. Compatible with Glock 20, 21 and 41. Will ONLY fit Generation 4 frames. Made of a high impact polymer, this insert is designed to keep out dust and debris as well as finish off the overall look of the frame. Allows for use with or without of the factory interchangeable backstraps. Finish/Color: Black. Type: Grip frame insert. Manufacturer Part #: PG-FI21G4.