Vltor Receiver Extension Storage Compartment Black. By installing this adaptor you will be able to install most any Mil-Spec dimensioned M4/AR-15 type buttstocks Five position receiver extension made from impact extruded 7075 T6 aluminum Hard coat anodized Positions are numbered and are visible through the witness hole on most stocks Watertight storage compartment that is accessible from the rear Receiver extension is Mil-Spec dimensioned and will only accept M4 type Mil-Spec buttstocks Package Contents: Vltor Receiver Extension Storage Compartment Vltor RE-2: Receiver Extension Storage Compartment Black