I had these for a while, but never really looked at them. Noticed what I thought was a scratch on the case, but is actually on the coin. Looking through the scope it is raised and extends into the head. Looks like thread or something, but probably die damage.
From your second image, it looks like it is raised to me, too. So, I would say that it is a retained strikethrough. It looks like Coolidge must have had head lice. Chris
I have something similar on a 2015 S Clad Proof Dime. Next to the Mint Mark. (not a good picture but it's there.. a fiber)