Luth-AR Thumbscrew - Aluminum Black. Fits between the buffer tube and the front of the receiver to securely lock the motor and its housing in place. Can be installed and removed without damage to the carbine. Our special high precision screws have nylon washers to ensure the best performance and reliability. If the buttstock is owned by an FFL or other weaponry then this part is required to complete the installation. Order this part along with your buttstock for a complete assembly. Specifications for Luth-AR Thumbscrew MBA-1 or MBA-3: Manufacturer: Luth-AR Color: Black Additional Features: Aluminum Fabric/Material: Aluminum Weight: 0.4 oz Features of Luth-AR Thumbscrew MBA-1 or MBA-3 For MBA-1 or MBA-3 Package Contents: Luth-AR Thumbscrew MBA-1 or MBA-3 Luth-AR TS-01A: Thumbscrew - Aluminum Black