Maybe it was mixed coolant, not ruled out, but it would have had to have been mixed at Brunswick, which at the time I thought unlikely. I dropped coolant and changed it (high pressure flushing the block) after finding this when I pulled the thermostats at just over the two year mark. There was no sign of any other corrosion anywhere else but the coolant was dirty.
Just ran a wire from one of the thermostat bolts to the negative battery terminal as a precaution after being advised by Gryphon Engineering that they believed it was stray current related. A year later it was still nice and clean.
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