Headstamp date really doesn't matter, it just depends on what the ChiComs had available when they made the ammo. If all they had was corrosive primers, that was what they used.
I bought an SKS Paratrooper when they were first being imported. It was before the ATF mandated that they couldn't be sold with bayonets attached and mine came with it still attached. I bought a 1000 rd case of ChiCom 7.62x39 ammunition at the same show, along with a bunch of stripper clips. IIRC, they were the yellow boxes, I can't remember for sure because when I got home, I unboxed all of the ammunition and put them on stripper clips for the SKS. The cases are lacquered steel and the bullets are copper washed steel. I distinctly remember that the boxes had the Norinco logo and name on them.
I shot the SKS and didn't worry about cleaning it right away because the boxes said the ammunition was non-corrosive. I got it out a few days later and the stem of the gas piston (behind the head, which is chrome plated, while the stem isn't) and the inside of the gas tube were covered with rust. There was some light surface rust on the outside of the barrel just behind the muzzle from the gasses flowing over it when the bullet exited. Luckily, the bore was OK because of the chrome plated bore. It all cleaned up OK because it hadn't gotten too bad yet, but it was perfectly clean before shooting it, so it was the ammunition. I'm just glad that I caught it early and that it was "only" an SKS. If I had shot ChiCom ammunition in a high-dollar firearm and found rust from supposedly non-corrosive primers that turned out to be corrosive, I would have been pissed. For me, anyway, after that, I consider ALL ChiCom ammunition to be corrosively primed, no matter what the box says.