It's a natural reaction of, I think, tin in the plating alloy with moisture in the air. You might also see some on solder joints. At near 35 years in storage it looks real good. In use, air movement carries it away and the end result is just a decreased brightness of the plating over time.
You could drop some desiccant packets into the cartons and then seal the cartons in plastic bags and it'll probably stop it from getting worse.