Is the square "eye" on the outer end of that casting tapered on the inside?
If it's tapered I'd think it's made to fit a wooden handle (like an adze.)
That doesn't make much sense, though, because swinging that huge piece of iron on the end of a wooden adze handle would be a pretty cumbersome task.
If it's not tapered, jakemac's suggestion above makes more sense; that it's designed to slip over a square steel shaft/tube and is some part of a device designed for wood splitting.