I have some HAM Radio amplifiers where the Heat Sinks are Blued and have had the same thing happen. When checking with the manufacturer, they said it was not an uncommon occurrence. They told me that it was due to impurities in the steel. Of course for this application I don't case, though it would upset me if it happened to any of my Blued Smith firearms.

Bob

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Sag Harbor & Manhattan, N.Y.