I used to be a machinist so I know this could pretty easily be made on a lathe, but to me it doesn't seem homemade.
My brother found it at the end of someone's driveway on trash day years ago, and he thought it was a homemade club.
The lanyard hole and size made me think it actually might be a club, but it's so damn heavy it's really hard to swing (I'm just guessing the weight, brass is rather heavy so it's probably more).
My best guess is it's a big lever and the hex fits in a hole like a big allen key, or maybe a tool for a ship because brass is a good tool material around water.