Your English comes off as not really being Russian, like Kyle Myers from FPS Russia. (I wouldn't be the least bit surprised to discover that you really live somewhere in Texas)
EP serial on stock plate is not an original feature on any New production sks from Any nation.
Paint on the stock plate is more likely a protective measure for upright storage in an armory rack combined with protection from skin oils from palm carry.
For long term storage, the Soviets used cosmoline. Ever heard of it?
I'm still waiting for the OP to show us a photo of the muzzle crown. If the paint was applied to the stock plate for storage, then surely the bare, crown in-the-white will have this storage protectant paint too, no?