Here is some better pictures, courtesty of Photoshop 9. (VERY easy to work with).
I don't think this erro is from an outside source. If it was heat or chemical, there would be some sort of trace on the edges when the zinc and the plating.


I don't think this erro is from an outside source. If it was heat or chemical, there would be some sort of trace on the edges when the zinc and the plating.























