I'm going with a temporary holder for a piece of square/hex stock metal - or a wrench for a square/hex stock shank. It may well be a very large chisel holder. I can also see it working against a fire hydrant if the valve stem is HEX or square stock. I know little of hydrant stems and I recall seeing different kinds. Maybe even a valve stem emergency handle of a general sort - think the valve handle being altogether GONE. I've seen "emergency kits" for the oil industry that included such things as square/round pegs of all shapes, brass shim material, wood limbs, rope, etc. that was used to "plug" or otherwise stop that precious black gold from being lost with a pipe busted, tank sprung a leak, or fitting cracked.
If you study the design (particularly the hook on the end), it is almost exactly the same (but a much larger version) as a Craftsman chisel holder. Their chisel holder design had that "hook" on the end to hold the chisel shank opposite of the tension screw. It reportedly allowed for a cold chisel to be inserted/removed more freely than what the "loop" design chisel holders would allow.
