A cautionary note for anyone who has a one-off shoulder stock like this example.
PLEASE remember that if the stock and the associated pistol are not specifically called out in the ATF website as being exempt from the provisions of the National Firearms Act of 1934, that the possession of a shoulder stocked pistol or the pieces to assemble such a pistol that isn't exempted is a violation of federal law because you have constructed, or can assemble, what is referred to as a Short Barreled Rifle (SBR)... a shoulder weapon with a rifled barrel of less than 16 inches from breechface to muzzle...
You can only possess a (non-exempted) SBR ---
IF and ONLY IF you have paid a $200.00 tax
and received in advance the appropriate approved tax stamp from the BATFE (Bureau of Alcohol, Tobacco, Firearms and Explosives).
Failure to obtain and possess the tax stamp can land you in the pokey for a long time... and a conviction will cost you your right to bear arms FOREVER.
If anyone buys the stock in question here, make sure you don't have a pistol that will attach to it until you have the stamp. BATFE agents are not known for their sense of humor in these matters.