Different opinion, based on the following site, I believe is correct. The SKS is subject to a whole series of other laws besides the assault weapons ban of 1994. These laws are rather gray, but it is possible an ATF interpretation may mean you cannot legally put a folding stock on an SKS.
From there, the following:
ASSEMBLY AND MODIFICATION OF FIREARMS:
SUMMARY: One may own a SKS in any form so long as the SKS is not a semiautomatic assault weapon or in a form that would otherwise be banned from import. Determining what is banned from import, understanding how to build a domestic SKS, and understanding the definition of semiautomatic assault weapon are more or less the universal keys in keeping your SKS legal.
When a person modifies an existing weapon, they are considered to be assembling a completely different weapon. US Code Chapter 44 Section 922(r) states that "it shall be unlawful for anyone to assemble from imported parts any semiautomatic rifle ... which is identical to any rifle prohibited from importation under section 925(d)(3) of this chapter". [I can find no legal definition as to what "identical" means. Identical in function rules out all detachable magazines on imported SKS's, but identical in form means non-military, aftermarket SKS magazines are completely legal since an AK magazine is not interchangeable with a SKS magazine.] 925(d) gives the Secretary of the Treasury final authority on what firearms can be imported or possessed in the United States if the firearm does not appear in section 5845(a) of the Internal Revenue Code of 1954. This code lists firearms that are particularly suited for sporting purposes and the SKS does not appear on that list. This means that the Secretary of the Treasury, who is in the cabinet and is a direct agent of the President of the United States, determines the fate of the SKS's import status. If the Secretary were to ban the SKS in all it's forms, it will no longer be legal to possess in the U.S., although most likely subject to some grandfather clause.
On July 7th, 1989 various military style rifles were banned from importation by the Secretary of the Treasury under the Presidency of GEORGE HERBERT WALKER BUSH. (I can hear you gasping even now!). On that day, he banned the "SKS style rifle with [a] folding stock, folding bayonet, or detachable magazine" from importation. This means that one may not import or assemble a weapon of this description unless it has TEN OR LESS of the imported parts listed under Part 178.39(c). [This still leaves open the question as to how all these Romanians are getting in with the the bayonet attached. C&R?] Those parts listed under Part 178.39(c) are as follows:
So, the law could be interpreted to prohibit putting a folding stock on an SKS based on an interpretation of the text. Not that anyone has yet been convicted of such a crime, or similar offense. From what I understand, there has never been one conviction vased on these laws, including the '94 AWB. Probably because they are worded so poorly.