Struck through Merc

Grease or could it have been planchet or just some odd thing that I dont know of.
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It doesn't look like a grease strike through to me from what I can make out
Lane
See http://www.doubledimes.com for a free online reference for US twenty-cent pieces
http://www.stellacoinnews.com/index.php/mercury-dimes-ch-6-1942/
Lane
See http://www.doubledimes.com for a free online reference for US twenty-cent pieces
down and to the right. Not correct in my opinion. But, then that's just my old tired eyes.
bob
PMD.
The surfaces off both sides are 'evironmentally damaged'
(a catch-all phrase), and the 'blob' is probably part of that.
If not, it's still something other that what could have occurred
during striking -