Fiat 780 not starting. Inhibitor switch?

I'm trying to get this old tractor started. The battery is charged, the lights work, the key turns, but there's nobody home.

Not even a click from the starter solenoid.

When I measure voltages on the starter there are 12V on one main connect, and another is to Gnd. A third is also showing 12V, and I would expect that to drop towards Zero when I turn the ignition switch, but nothing happens.

The wiring diagram has a 'Starter Inhibition Switch' between the starter motor and the ignition switch. Where is that? Can't find anything about it anywhere.

It's my father's old tractor, and he is no longer present to admit to whatever has been happening to the wiring in this tractor.

I need to get it started so that I can find out where it's supposedly leaking oil, and if that can be fixed, I really need it to run a hydraulic log splitter. Lots of deadfalls and broken trees after Hurricane Amy that really must be cleared. And big heaps of branches that needs to be pushed into the woodchipper.