cleaning up the slant six head

SSD,
Review his pictures again. I just did. I see old gasket material and I see what very clearly looks to me like machined iron cyl head surfaces that are being easily gouged with a screw-driver. Clearly something attacked that iron.
Have you ever soaked iron or steel in vinegar? Just because you can consume it doesn't make it benign to other materials. It removes Cadmium or Zinc plating with aplomb. Those aren't the same as iron, but until scientifically proven otherwise I'm content with the explanation of events.