Was the cartridge stored in the original box? Older Prvi boxes used cardboard dividers, while newer production has plastic dividers.
It is possible that the cardboard divider was not made with acid-free pulp. A lot of Swedish 6.5x55 M94/41 ammo is tarnished because the boxes, up until some point, were made of acidic cardboard. But these cases are tarnished, while yours is fully corroded.
Just guessing: maybe the corrosion is from a sweaty finger tip that touched the cartridge during examination, and the cartridge was returned to the box.
Use a non-metallic scouring pad and clean the corrosion off. If the underlying metal is pitted, pull the bullet and powder and use for a reload.
Since the corrosion is close to the base, I would not shoot that round.
I cleaned the tarnished Swedish 6.5x55 with a pad and shot it, but there was only discoloration, not pitting under the tarnish.
I like "ammolab's" theory. Maybe a drop of bore cleaner fell on it?
My favorite bore cleaner is Ballistol. It controls copper fouling, i.e. it may not be as aggressive as other products, but if you leave the bore "annointed" with Ballistol, a clean dry patch pulled after some time of storage will show green streaks, I bought a pump oiler to use as a dispenser for my Ballistol. Stupid me did not realize that the brass pump housing might be a problem. Sure enough, when I went to squirt some Ballistol onto a patch, it was discolored. The Ballistol was eating away at the brass pump housing.
Lesson learned: I got another oiler, but this one I made sure had steel or iron pump parts.
Were these rounds randomly distributed among the 7 1/2 boxes you have, or were they concentrated in just one or two of the boxes?
Did you at some point open the boxes to inspect the rounds and pull them out of the dividers? Both "ammolab's" theory and mine seem valid. If you never opened the boxes or touched any of the rounds, then possibly some fluid leaking from the packing machinery got on the cases.
That's the oddest corrosion I've ever seen!
Almost perfectly circular & even the color looks a bit "off" from the usual greenish-blue. The edges are incredibly well defined too. Plust is ONLY in that one perfect circular patch!
This is far more typical. Random shapes & "soft edges": http://www.superiorpawn.com/ammo-disposal/1.jpg
(c) Superior Pawn, Virginia Beach