Not much to add, really.
As explained, the side plates were blue when they left the factory. With their new production, Mauser started the use of cast and sintered small parts. This was done to make production more cost effective. It seems that the different composition of the metal caused the blueing to react in a different way over time. (but not always, I have NIB pistols that are as blue as when they left the factory).
Which makes me wonder whether heat and/or UV exposure play a part in the process.
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