SN and arsenal markings.
You do not say who made your SKS.That helps and usually the russian made SKS rifles will have the year of production stamped on the receiver cover.I am not so sure on the Norinco made ones.
Markings.
The RUSSIAN SKS may be date marked on top of the receiver cover (properly called the rear housing) , along with the symbol of the arsenal that made it.
This is purely opinion on my part, but the SKS was created in Russia, and made there for a number of years before the Chinese began manufacture. The Russian SKS tends to have better quality machining than the Chinese. Both are good, but the Russian is the original, and the better built rifle. Examine one of each side-by-side, and see what you think.
Russian
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By the markings. Send photos to my email address if you wish to.
Very similar, minor differences,
Is it marked with Chinese characters or in the Cyrilic alphabet? <><><> As above. In addition, MOST Russian SKS rifles had blade style bayonets. MOST Chinese rifle had a spike type bayonet. But marlings are the defintive answer.
Sorry- these were made in several nations, and there is no way to tell age from a serial number alone.
Importer name
they are about $100