Looks like a thread/fiber. I found one like that once on a business strike coin:


You can see where the thread/fiber was and still is on the coin. I've seen evidence of a thread even on proof coins, but the business strike coin was the first that I actually found the cause.



You can see where the thread/fiber was and still is on the coin. I've seen evidence of a thread even on proof coins, but the business strike coin was the first that I actually found the cause.























