I have had that on some of my coins in proof and mint sets, I had to look under magnification and found a extremely small piece of debris on the coin right in the center of the spot. I believe they are called carbon spots. you can remove the debris with the end of a tooth pick but the spot will always be there.if you remove debris the spot won't get any bigger but wont go away.if you leave it the spot will continue to grow. I have had to open about fifteen proof sets and about four mint sets to stop the damage.I may be wrong about not being to remove the black spots and if I am I'm sure some of the experts here will chime in..
its a carbon spot on the slabbed 44-D yours could be the same but I see a bit of green, is that true or just the pics? If true, verdigras or again, a form carbon forming.
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