I do not think it is a plug of any kind. If it were, wouldn't it be more tapered and solid with no rings. What would be the point of rings on a plug. Not sure about case shot either as someone mentioned, wouldn't it be more efficient to just make them round like they did. What would be the point of making case shot shaped like Minnie balls but larger. Were all known specimens located in same area or in numerous areas. I believe it may be some sort of fence post ornament or some other decorative piece. Could it be the projectile portion of an early small caliber anti aircraft round, don't laugh just throwing out ideas.