56S is the fixed-stock version.
56S-1 is the underfolder.
56S-2 is the sidefolder.
I'm not the most knowledgeable person on AK-47 variants, but I do not recall the Norinco 56S-2 having a folding stock like the one pictured. Is this some sort of conversion that someone has done? Still, it's a great price for a pre-ban Norinco, especially when you add in the black Bulgarian furniture and AK-74 brake.