I go with Ivan on this.
The square hole is where the steel held the ball and the slot is where the chain links went through and on up to the ship/barge.
If you look closely you can see where the chain links have wallowed out the slotted end into an almost figure eight shape.........
Exactly like a chain link would because it has no center.
I think if Ivan posted a pic of one, it'd get a green check.
But what the heck, I thought it was a worn out cider millstone too.
I still think Ivan is right.
Sky Pilot