This is a Ergo Grip Rail Panel, model 4379-DE.
ERGO's 18 Slot AR-15 Textured Slim Line Rail Covers (3-pack) protect unmounted rail areas from damage and shield the operator from sharp rail edges. Its rubbery grip surface improves weapon control and is 6.8" long. Can easily be trimmed for custom fitting. Low profile and fits all 1913 Picatinny rail systems.
Features:
Protects from damage and protects shooters from rail edges
Molded in Santoprene to provide a low profile
Easily trimmed for custom fitting