I was thinking it looked like an oil reservoir, but while I've seen that type, I've never seen one on a Bronco I don't think.
On a tractor maybe? Could be a PO scavenged it for a custom "oil pressurizer thingy" for cold startups? Or it was for a hydraulic auger option (post hole digger) on the back, or hydraulic snow plow contraption on the front of a Bronco maybe.
The augers were usually at least partially PTO operated, correct? Maybe this was for the other part, and there was a PTO operated hydraulic pump?
Good thing to hunt down. Any chance or more pics blu?
Paul