Kikir, SKS rifles were made in many arsenals around China. The most common arsenal is "26" in a traingle. Using the serial number of arsenal "26" rifles you can determine what year it was produced. During it's period of manufacture there were many changes made to the rifles. At first mostly milled parts were used, and barrels were screwed into the receiver. Later stamped parts were used and the barrels were attached with pins. There were many other changes, which are to numerous to talk about here.
Also some early Chinese made rifles are call "Sino/Soviet rifles". The Chinese obtained the machinery to manufacture SKS rifles as well as a supply of Soviet made parts from the Soviets. These Rifles are called "Sino/Soviets" because of the use of Soviet parts on otherwise Chinese rifles. These rifles also had their own serial number range. These rifles also either had or didn't have parts or markings which would indicate they were made during this time period (such as a blade bayonet instead of a spike)
Some collectors collect rifles from different arsenals which have different receiver markings.
The true authority on all things SKS is a fellow board member named Howie Bearse. I hope he will chime in and further illuminate you on these cool rifles. Also check out the following web site additional info on SKS rifles.
www.simonov.net