North Vietnam set up a small arms factory north of Hanoi (12km north of Yên Bái, to be specific) with the assistance of Chinese factory #296, starting in 1959. The goal was for this to become a substantial arsenal for both repair and production of arms, including barrel-making and forging capabilities. It would never reach this potential, but it was able to assemble about 6,000 SKS rifle between 1962 when it opened and 1965 (when it was shut down by American strategic bombing.
Serial numbers on these rifles begin with a two digit code identifying the year of production (62 through 65) and then a 4 digit sequential number. You can see an excellent database of known examples (as well as North Korean and East German examples) on SKSBoards.