Well, if you want to sell it, I'll give you 99 dollars for it, 20 dollar profit. Can't beat that!!! Seriously, beautiful gun and there is no way to date it. Can only date factory 26 models. A DB is a more of a collectible varient than a standard chinese sks. Probably worth around 10-15% more. That one is totally complete, all matching with the wood, factory mag etc. I'd keep it just because it is in excellent condition. You can get an AR anywhere, but not a nice 20 yr old sks for 79 dollars
If I remember right 56 is the first year that the chinese started building the sks. Hopefully someone with a little more knowledge will step in and give you a little more helpful info