I hate to say it, but have you ever considered Carbines rifle was like martins when it came here and someone found out what it was and added the white painted Arabic numbers? Personally I find it interesting that EVERY norinco marked rifle is in new condition minus Carbines. What are the odds that all these guns came in and the new ones were for some reason separated from the well used examples specifically to mark them norinco? I would say not likely at all....
All I can say... your asking why Century Arms did something... the king of doing weird unexplainable oddball sh*t through he years. Even better.. your asking why they did something way back in the 80's, when they could get away with alot more. Separation and remarked... I could easy see it... hand picked excellent condition ones, make more $$$ on them, next up - used lower grade ones. If there is a reason, I'm sure it was financially motivated, thats what drives this world, that almighty dollar. Are you gonna sell a really nice minty condition one for the same exact price as a used, half nasty camel crap soaked one, I would imagine not, if they did, they really are as ignorant as I thought. Change the stamp, make people think it is something different...little classic retail deception... not everyone knew about stamps, It was, hey this one is better shape, a little more money and has a different stamp, so it must be different than that one.
Realistically.. if marked Norinco China or Made in China.. they are still correct, as they are of Chinese origin. It could have been.... opps we forgot to stamp these laying over there on a pallet, while they were in the middle of another run.. the current stamp is close enough..lick'em stick'em and send them on their way. Kinda like why go from stamping the barrel, to stamping the receiver and then back to stamping the barrel. I would think the receiver should have been import stamped....considering the receiver is the actual ATF controlled part... any jack leg can buy a barrel, but a receiver requires an FFL transfer.
I say... it's a 50/50 guess. Speculation at it's finest. flip a coin..