No country which manufactured the SKS gave it the nomenclature of M21. The only country which applied an "M" number which I can think of offhand was Yugoslavia, who designated theirs M59, and M59/66 for rifles which had the 22mm grenade launcher fitting. Yugo SKS rifles can go from $150 - $500, with the latter price being for an unfired, unissued rifle. SKS rifles in general can go from $100 - $1300, depending on the exact model and country of manufacture (Vietnamese and North Korean rifles have the highest value, being the most rare). You can send photos of the rifle and markings to my email (listed on my profile page), and we can determine the exact model and value from there, if you wish.
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