It's a little more complicated than that. Many Chinese SKSs (the ones originally manufactured for military use) can be dated through their serial numbers, but the BATF refuses to acknowledge that so they classify all Chinese SKSs as non C&Rs regardless of their date of manufacture.
Also, many commemorative guns and guns with historical value are added to the list every year. I suspect that it is often political cronyism so that someone that supported a politician can own a gun in an area that restricts non C&Rs much more heavily.
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