Some SKS information to add:
There are two eras of M21 production, the first run is thought to be from the 1960's and are, as ardent collectors describe, "new porduction for foreign military use". China supplied these guns to the North Vietnamese as a clandestine attempt at disguising the origin of the arms. These rifles do not have the typical triangle arsenal mark, or the Chinese characters that are found on other military production models. The serial number begins with an "M21", which is directly followed by "No." and then the serial number. None of these specifically marked guns have been documented to have been imported to the U.S., leaving the only probable means of entry into the country would come from North Vietnam capture and bringback.
The second run of M21's appeared in the late 1970's and possibly the early 1980's. These were extremely high quality guns, made with the same basic features of the late factory /26\ guns. The handguards came in two patterns of ribbed brown or tan plastic molds, and are commonly referred to as the French Tickler type handguards. These serial numbers also had the M21 designation, but also had another identifying block of numbers such as (0296), (416) or (016), IIRC. These were import stamped guns, brought to the U.S. for commercial sale.
Here are some pics of my import marked M21 (0296). These sport the French Tickler, and are fairly common. Exceptional finish on these guns, as well.
Close comparisons of both versions of the M21 indicate that they were most likely made at the Jainshe Arsenal /26\, but not stamped as such.
I've been waiting for YEARS to score an early M21. And to find one that was most likely a cache weapon is almost incredible. Hence, I am STOKED!
No words of wisdom come to mind at this time....