Shoot......guess the anomaly around the '1' wasn't a rub-a-dub, but aliens........excuse me while I get my head out of Agatha Raisen.........the wife wants to watch it tonight.............yuck.
It appears to me to be a slightly deteriorated die. The one on the striking die would be a depression, and the field would be the top flat area of the die around the "1". Maybe a rust spot .