It's not that you need to have a certain amount of american made parts on the gun,
You cannot have more than 10 foreign made parts on a rifle/shotgun with a pistol grip. This is the reason saigas were imported with the funky stock, and the reason norinco AKs came in with thumbhole stocks. Century and their family of WASRs are built 922(r) compliant using a bunch of US parts - i believe they stamp receivers in the US and assemble the guns out of buckets of parts kits - country of origin of furniture and stuff being the "country" of the gun.
The ATF technical department decides what the breakdown of the gun is, and what parts are parts and what parts don't even count. Kind of the same way they decide what half is the receiver on what gun.
I believe the AK family breaks down to like 14 parts
Trigger, hammer, disconnector, stock, pistol grip, bolt carrier, bolt, barrel, receiver, front handguard, magazine, mag spring, mag follower, gas piston
Interestingly enough, the saiga-12 has a piston and an op-rod, but the op-rod is not a separate piece. The puck piston is the piston.
Im pretty sure this is right, someone should google it though.
Out of this list of parts, if you make your gun subject to the 922(r) restrictions by adding a pistol grip stock- even one of those nonsensical tapco rearward pistol grips, you cannot have more than 10 foreign parts. The magazine has 3 parts, thats why you can't really use it after converting. Thats the same reason tapco took the time to stamp usa on their triggers and why century aks come with tapco triggers. Its ridiculously confusing