Intriguing! I found one of these cast iron "balls" at the site of a Civil War battle. It is larger, about an inch and a quarter, this appears to be about 7/8". True grape shot were pretty large, approaching two inches and used primarily in naval cannons, the heavy ball being able to penetrate the wooden hulls of warships of that era. Far be it from me to dissent with the Historical Society's definition and they are probably right, but had I found this I would think I had found a cannister shot.