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Gases can cause this too. No telling. Acid in some paper products. Whatever it was the plastic trapped it in.
I hope it cleans-up. Might start with Evapo-rust. A glass of Coca-Cola?

That razor looks like fairly recent production. Bar codes and such. I would think it modern plastic. Surely not Celluloid.
Bet some moisture got in the plastic and rose to the top and could not...