The GC Greek HXP (Hellenic Xartridge Co) was stored 40 odd years in phosphated steel stripper clips causing some galvanic corrosion to the outsides. That is, it's NOT an inside-to-outside corrosion, but rather external and (mostly) superficial.
The black grainy gunk around the cartridge rim is the black phosphating from the stripper clip that's come off when GC (somewhat greedily) separated the stripper clips for separate sale at $20 for five.
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