I was thinking the same about it being a grenade but thought it would be pretty heavy to throw any distance. It could be any type of fragmenting cannonball, mortar, grenade, artillery, cluster shot, ect, but I chose the term cannonball because I just thought it best described what it looked like at first glance. In fact, I was even thinking it could have been a topper for an old iron fence. I have been looking at hundreds of google images and books this week and see lots of stuff that is close, but nothing so far that jumps out as definite.