Is a post ban AK (ala those sold by Century, etc) a "US" rifle?
If it is, would it be legal to put a folder on it?
If so, does it have to be a US folder?
If one puts a Chinese folder on it, could I increase the US parts count
elsewhere, to keep it a "US made" firearm?
Maybe, Centruy Arms imported several models of AK rifles from Egypt(1997-1999) and Romania(1998-present day). All the Romanian rifles had a US made trigger,hammer,sear,and gas piston added and the rest of the 10 parts were imported. So they are considered domestic produced rifles and legal to add a underfolding stock to. If you add a imported under folding stock you will have to add another US made part to keep the imported parts count down to 10.
I hope you are aware that you have to remove the rivets of the rear trunion and the rear trunion. Weld up the rivet holes in each side of the receiver. Then drill new holes for the rivets of a underfolding stock rear trunion and the underfolding mechanism. Install a new underfolding stock rear trunion,rivet it in and install the underfolding stock. Then weld a steel plate on the back of the reciever.