You have a lead ball that was cast and lost before the sprue was cut off. When casting lead balls a person will pour a bunch before cutting the sprue off. This is because the fresh cast ball is plenty hot, and the mold also gets plenty hot, so you make up a bunch of balls, let them and the mold cool down and then cut the sprue. Some how or another that ball you found was lost before the sprue was cut off.
In the photo, the ball mold would be a type used from the 1700's until today. I probably have more than one myself, among the various sizes of bullet molds I use. In the photo the balls between the handles still have the sprue on them, and the ones to the right have been cut using the cutter located on the mold.
This is a different style bullet mold, this one also dates to the civil war and the type is still made today. This one is a colt pistol mold and casts a conical bullet and a round ball in the same mold. What we are interested in is the sprue cutter. On this one the lead is poured in through the holes, then let to set up, which doesn't take long, then the plate on the top is tapped sideways, cutting the sprue off and leaving a flat place on the ball, where the previous mold in the other post leaves a pinched cut.