Congratulations, but I doubt that these are so old.
Make sure that there is no cosmoline in the bolt, as some SKS have suffered very nasty 'slam-fires' when the bolt slams the round into the chamber.
One guy died at his rifle range due to a slamfire, according to "the Sacramento Bee", several years ago.
You might also know that the Chinese wood (unlike the Russian and Yugo wood) is considered soft and fairly easy to cut, but after a bad mishap, stocks can often be found on 'FleaBay', or these websites.
There should be a decent disassembly video at "Surplusrifle".
Seeking some more really cheap 8mm Yugo surplus in the Memphis area.