Are you sure it is metal 'filling' ? It appears that the top of the massive stain/chemical reaction on the top and bottom has a path right into it. I think it could be a stain from a solution that flowed into it,but after the first photo , the focus and resolution is bad that I can't not tell. You can see similar in bay 5, and more like a stain/chemical rxn.
View attachment 443744 I tried mineral oil and acetone the "stain" didn't come off. It is 3 dimensional. It is raised metal.
The acetone and mineral oil didn't remove the stain from the rest of the coin either. So it could be a substance that, when hardened, won't be affected by solvents. I suppose it could be a die break. I've seen lots of them in the corners of bays, but never anything close to this. One surefire way to find out. Scratch it and see if you see shiny copper underneath. The risk of course, is that if it is a die break, you'll have an error coin with a scratch.
