It was not original on that tractor. My guess is that a previous owner had a problem with the electrical system where the engine wouldn't shut off with the key and so installed that. Does the key switch shut the engine off? If so, then some later previous owner probably fixed things right and disconnected that thing from the circuit.

 

The tractor is apart right now, but as I recall it did shut down with the key.
Thanks for the input, I'll try to figure something else out to do with the hole where that switch was!