- The image is not large enough for me to arrive at a definite
conclusion. But it looks to me like a spot of subsurface corrosion
that has lifted up the overlying copper plating. The mintmark is an
area that is prone to such corrosion, because, for whatever reason, the
plating tends to split or develop microscopic gaps here, and that can
eventually lead to oxidation and corrosion of the underlying zinc.
--- In firstname.lastname@example.org, "Tom Bunch"
> I uploaded 3 photos listed as "cud" of a mintmark and what appears to
> be a stack of "D"s beside this mintmark.
> What is this?