Originally, under the 1968 law, a gun was a C&R ONLY if a specific model or a specific gun, by serial number, was designated as one by ATTD (as it was then). And a gun was designated as a C&R ONLY if an application was made and approved to so designate it/them. That is where the old lists came in, with some categories (Blech pistols from Lower Slobovian contract...) and some specific serial numbers.
The 50 year rule was part of the newer law but the old lists were not changed.
In any case, auto weapons could be designated as C&R but still require transfer approval and tax (Form 4). That applies only to guns - something like a carbine M2 kit, by itself, is not a C&R and a C&R license holder would be the same as anyone else in buying it.
I know of no blanket ban on the SKS being a C&R or any exception to the 50 year rule. But if BATFE cannot verify the date of manufacture, the 50 year rule becomes moot. Also, China has been made (by George H.W. Bush's executive order) a special case for arms import; neither handguns nor "assault rifles" can be imported from China, even though similar ones can be imported from other countries.