We all thought it was tape. The one other thing I can think of is that it's a black mark from an old coin counting machine. These machines had a black rubber "tire" molded onto a wheel. The wheel would turn very fast and grab the coin and shoot it out as it counted it. I've seen coins in OBW rolls that had these marks. It they didn't have the machine set right, they would leave rubber marks on the coin like car tires and pavement. The rubber would wear off, but the damage to the coin wouldn't. That's all I got.