This is the Command Arms PRFCS Polymer Picatinny Rail for Collapsible Stocks and Adjustable Checkpiece (ACP). It's solidly constructed of rugged fiberglass reinforced high-density polymer to provide a lifetime of heavy use.

Features and Specifications:
Manufacturer Number: PRFCSACP
CAA AR-15/M4 Stock Picatinny Rail Right Hand Shooter Cheek Rest Polymer Black
Picatinny rail for most original M4 style stock
Rail to support adjustable cheek rest
No gunsmith required
Mil-standard
Eligibility: M4 style tube
Materials: Made of Polymer
Weight(oz): 4.94
Color: Black

Fits:
AR-15 M4-style stocks