It's 1 jan 1899 or earlier, not 100 years, for "non-firearm" antique status.
Remember "Innocent until proved guilty"? The ATF has to prove that a gun was made less than 50 years ago. Otherwise it's C&R. If you have any authoritative way of dating its manufacture, if pre 1962, then its C&R, regardless of any markings on the gun, and the ATF doesn't have a case.
If anyone cares enough and reads Mandarin it may be possible to get SKS factory records. That's assuming they aren't military secrets. otherwise everything in China is for sale. This probably would have been done if SKSs were still being imported.
BTW the younger Albanians, Yugos, etc were classified C&R thanks to being certified by a public museum curator as being of significant special interest, etc... Unlike the 50 year rule this isn't automatic and a firearm accepted via this process as C&R by the ATF then goes on the list, although most of the firearms listed have aged to beyond 50 by now.