The pix posted is NOT the subject shell, but the earlier find. No pix of the subject shell.
Even a Camp Vrednenburgh existed for the Civil War on or near the grounds encompassed by Monmouth so undoubtedly that could be a possibility. But I cannot believe they would use metric. Hence I think should be even later. Posiibly some of the last.
Unless they titally misnomered it and it is nit a “ball” at all.
This is only going to be a wild guess, but I think someone owned a cannon before they were federally regulated. Following that theory, they were just having fun shooting their cannon. Before you dismiss this, just go look on YouTube at the cannons that people fire now. Additionally, when I was growing up in the 1960s, we had a neighbor that had a civil war era cannon and he shot it from time to time using real cannon balls. Needless to say, we kids were always there to watch it.