I found it in an undeveloped park which had been a huge gravel pit type area. It was mounted in a burned up piece of wood. Couldn't resist taking that home.
Evidently you drill an axial hole in the end of a 2" Dia pole and a transverse hole thru that. The threaded shank goes in and the crosspiece holds it. The slot helps align the crosspiece. It's sure a stronger rig than a simple screw eye.
I guess you could use that for anything you want but must have had a specific purpose.