They are black lacquered steel cases, are they?
If so, it's the Chinese factory 14, unknown site, wich also made tracer (green) ammo (91 too). If they are not tracers, they might be soft steel cores.
If it's brass cases, it can be PPU-made, they are known to be using the 14 + year code, but I never saw any 7.62X39 made by factory 14 (PPU).