Well, sadly the Russian stock sets that do make it into country are "grey market" imports.
They are not officially imported, as the ATF would disapprove an import permit for "AK stock sets" from Russia. They are a component part of a firearm not covered by the US/Russian arms agreement. . You might have some luck with calling them "Saiga" stock sets. They make it in thru using some very wide stretching of the $100 import rule. I know one guy was able to import some stock sets legally on a Form 6, but eventually the ATF told him no more.
They are not officially exported either. While they do clear Russian customs, it because of the "extra customs charges" and not because its 100% ok. The Russians are forced to honor the US/Russian arms agreement. Other items are considered dual use. Izhmash sells the parts are Saiga spares, while in fact they are original Soviet military AK parts.
I have talked to Andrei, and the man behind him. Both have said they cannot get any RPK stuff. Molot doesnt sell spare parts (they tried) like Izhmash does, and there are none in the Army depots they buy from.
If they were 100% legitimately imported and exported, wouldnt some big company like K-var or someone be selling stock sets by the container full? The stuff is over there, the Russian military still sits on large numbers. The Russians are willing to deal, but as one European gunbroker told me "only by the thousands"
The K-var stock sets are Russian via Bulgaria.