You might be right and the cover is made of copper sheet, in which case it must be very oxidized to have this color, which would diminish its role of ESD protec

You might be right and the cover is made of copper sheet, in which case it must be very oxidized to have this color, which would diminish its role of ESD protection. Wasting so much copper sheet for a cover that will be dumped during the PCB assembly is not wise, but in the Soviet Union they were rather careless about wasting materials, so this would not be unusual. For other early MOS ICs or transistors, which were similarly sensitive to ESD damage, the vendors did not use so big covers, but only a metallic clip made of thin elastic wire, which was applied on the pins and which pressed on them until removed, due to its elasticity, providing thus a short circuit to avoid discharges. However any such metallic shorts raised the cost, so they were replaced with delivering the ICs either contained in tubes made of conductive plastic or inserted in black sponges made of conductive foam.