Pre-ban is really a meaningless term as it relates to firearms not subject to the ban. Since the SKS, in original configuration, does not use a detachable magazine, it was never subject to the “assault weapon” ban. That’s why having a bayonet, a grenade launcher, or adding an after-market folding stock is a non-issue. However, even if the SKS was manufactured in 1953, if it was never configured as an "assault rife" prior to the 1994 cut-off, doing so today would constitute the creation of a new "assault rifle", and would therefore run afoul of the current ban.
Of course there are many who will say that it would be nearly impossible to prove that such a weapon was never in assault rifle configuration prior to the ban, but I'd hate to have to argue that in court, unless you have some type of evidence.
As stated previously, the SKS's are most likely all C&R eligible, if they have been maintained in their original form. But that has nothing to do with the ban, merely as to whether they can be purchased consistent with other C&R weapons.