Iíve gotten several questions on the stock and itís adapter. This is what it is; Exile Machine AR Shotgun Stock Adapter for CA AR-15s https://www.exilemachine.net/Headbutt.html
I tried to find a site selling them and the only one I could find is their Amazon link. Exile Machine Headbutt SGW-515 AR/Shotgun Stock Adapter - Black https://www.amazon.com/dp/B01MUKA3SB..._rbGrBbBNWCWCG
I donít know but am unsure if they are still being made?