Scorpion Evo 3 S1 Pistol Arm Brace Kit. – ENABLES THE ADDITION OF AN AFTERMARKET ARM BRACE OR OTHER PISTOL BUFFER TUBE ACCESSORY – ADAPTER IS MADE OF 6061 T6 ALUMINUM FLAT BLACK ANODIZED – BUFFER TUBE MADE BY KAK INDUSTRIES SPECIFICALLY FOR FITMENT OF AN AFTERMARKET ARM BRACE – THREADS ON ADAPTER (1-3/16×16) ARE STANDARD TO ACCEPT ALL AR-15 BUFFER TUBES/RECEIVER EXTENSIONS – IF USING A BUFFER TUBE OTHER THAN THE SUPPLIED KAK INDUSTRIES TUBE THERE ARE TWO THREADED ALIGNMENT HOLES (10×32 THREAD) FOR TUBES THAT USE A NOTCH FOR INDEXING. A 3/16 HALF DOG SOCKET SET SCREW WOULD BE USED FOR PROPER INDEXING. – GUNSMITH INSTALLATION IS RECOMMENDED