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That takes quite an eye to distinguish!
Very easy to spot for a seasoned "mule hunter"

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Are you tempted to treat the verdigris?
edit: actually on my PC screen it looks more like the colour of PVC chlorine attack.
Don't want to do any more harm. Then again I don't want it to spread.
I'd have to try out any verdigris treatment techniques on less important coins first.
PVC chlorine attack usually wipes of with a bit of effort. This doesn't.