The first Norinco SKSs were made in 1956. If you add the first two numbers in the serial number to 56, you get the year of manufacture. For instance, my Norinco SKS serial number is 23014xxx. Adding 23 and 56 I get 79. The year of manufacture is 1979. That's important because what's been said about pre-89 Norincos is true. A bayonet can legally be put back on. I actually removed mine, but I doubt BATFE will be concerned with that modification I did. 027 should mean 1958 manufacture, but even using 27 instead of 2, 56 + 27 = 83, or 1983 manufacture, still pre-89.
The 21 by itself is confusing. Mine is stamped M21, and that code says that Norinco produced that SKS expressly for export. For commercial reasons. It was built and shipped as new. This is opposed to a code such as 0406 (and many others) that designate a SKS as having been produced in a Chinese arsenal for the Chinese army. It may have even been rebuilt before export.
A great site for SKS info is
http://www.websitetoolbox.com/tool/m...fle?forum=4870
Probably the best advice given here has been to call BATF.