Reasons for a lead ball that don't include artillery...,
Ballast for a ship (but round might roll back and forth a bit in a rolling ship's lowest point.)...,
A counter weight (Colonial Williamsburg sports chains on the back of the gates to their properties that have round ball weights that then act along with the chain to close the front gate to the front yard automatically when the gate is released.)...,
15.5 lbs modern is ALSO 1 Arroba under the Old Spanish system which was used in Mexico and South America, and it's also 1/4 of the Old Spanish Quintal (give or take an ounce). So..., if it was dug up in an area that traded with Mexico or was once part of Mexico, perhaps we have a solution. ??? A weight for a large grain scale? (odd though it's not marked with a large "1" or an "A" or "1/4 Q" or something, eh)....
LD