Guys,
Below is a very detailed response to this question posted by a guy called "REDSTAR" in the SKS forum on AK-47.net:
"Here is the problem Doug. You're merging the evil features test for semi automatic ASSAULT rifles with sec 922(r) which deals with the assembly of semiautomatic rifles (note, rifles, not assault rifles). Although the no more than 10 imported parts assembly rules apply to semiauto assault rifles as well, the reason why it would be illegal to put a folder on an SKS is because it is illegal to assemble any rifle that is prohibited from importation under s925d3, which is the sporting purposes test. This test bans rifles from importation if they have only one evil military feature, detachable mag or not. The folder you want to put on the rifle would automatically put it in a configuration that is not importable (because of the folder, and bayo lug if it had one).
The only way to configure an SKS with a folding stock would be to only have 10 imported parts on the rifle which is impossible because there are 16 or 17 parts of the SKS that are on the list and few if any of them are US made.
I know this is confusing so I'll try to summarize:
1) The 1989 ban does not apply in this particular case.
2) The disqualifyer is s925d3 which disallows importation of non sporting rifles ( military features).
3) There's no way to use s922(r) to your advantage because there is no source for all the US parts you would need.
You may question what about Romanian and Russian SKS's that are importable. Nope you can't do them either becuase they have to be imported in original military configuration. Those are governed under another section of the law.
A heads up. The ATF uses your specific example to show how a violation of s922(r) can occur, so they're well aware of all the folders for sale out there! To see it, go to the ATF web site, pull up the 2000 Fed Firearms Reg Ref Guide under "publications" and look at page 125 third column, start of the first new paragraph."
REDSTAR also posted some additional information in subsequent posts, so if you are really interested in this issue, check AK-47.net.
Doug