That's an easy one! One out in my tool box right now!
It is what is referred to as a "Fish Tail Wrench"!
Note the Euclid colored paint on it? They were used to adjust the rear wheel bearings on a Euclid off highway truck. There is a threaded hole in the rear wheel hub that you put a bolt through one of the holes and the "Fish Tail" part drops into the notch in the bearing adjusting nut then you turn the wheel and this tool turns the wheel bearing adjusting nut.
I don't have any old Euclid service manuals at home but there was a "spec" for how heavy a guy you had push down on a 10 foot pipe to get the "proper" torque on the wheel bearing nut!
Here is a very crude drawing of how it would be used. Green is the wrench and red is the wheel bearing adjusting nut. You turn the black wheel which in turn turns the wrench tightening the red nut.
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