I’m sure it workes well but… - DoubleStar Folding Stock Mechanism with Integrated AR-15 Stock Interface w/o Boss

I wanted to install a folding stock adapter to my AR pistol and settled on this one. Once I received it I could not install it because nowhere in OpticsPlanet website does it say that a second part is needed in order for this adapter to work. It makes no sense to sell only one have of an adapter. SMH, so beware.