Richard Godin adds:
"I found three tags. USMC tags were made out of round metal, thick and heavier than the army/air force/navy ones. A friend of mine told me that it was a 'heavy water' process. I believe it was an anodization. The guy dipped his finger into a greasy substance, heavy water or whatever, then the tag was dipped into an acid. The acided would eat around the print for a very short time, then dipped into water and washed to stabilize it. It could only be done with USMC tags as they were iron, others were aluminium, some sort of stainless or brass."

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