Hi, I do a few "whatzit" a week for various discussion boards. Is it possible you have mixed up "diameter" and "circumference?"

If I just look at the picture without using your dimensions, it looks just exactly like a WWI-era "one pounder" projectile used in small fieldpieces employed by may different nations including the US. Most of them contained some kind of explosive, sometimes even black powder, so be careful.

If you measure across the flat base, the extreme measurement should be right about 1.457 inches, close enough to 1.5 inches.

If it is 1.5 inches, I'm sure you have a 37mm projectile, possibly intended for the M1916 gun.