This is a Ergo Grip Rail Panel known as the WedgeLok, model 4332-4PK-DE.
ERGO Grip's 7-Slot M-Lok WedgeLok Rail Covers (4-pack)" improve your grip comfort and performance. The WedgeLok design snaps securely in place to eliminate movement or shifting of the grip panels. This provides you with a solid base to apply grip pressure on the forend without sacrificing comfort.
Features:
7-Slot M-LOK Slot Cover
WedgeLok Design
Santoprene Construction/Heat Resistant
Low profile with aggressive texture
6-1/4 Long and 5/8 Wide