It *could have been* an attachment for a heater hose to run hot coolant to a radiator in the cab at one point. If so it appears to be blocked off now. Just a guess.
Dad and I need to figure out how to do that in the '29 Chevy. There's a period heater in the car but no plumbing whatsoever.
Go to a good auto parts store and ask for a couple of Four Seasons heater hose fittings. One goes into the side of the lower hose for the cool water return and the other needs to go below the thermostat or in the top hose if it doesn't run a thermostat.
Best drill and tap into the cylinder head and put a thermostat in the water outlet at the top front or in the top hose.
Most early Ford V8 stores also sell them. C&G Eraly Ford Parts does.
Tap a hole in the lower hose with a gasket punch. Use the punch the next size smaller than the actual fitting is.
Could it be to provide water to prime the pump? JAWAG. Dave
Looks more like a valve handle then a pipe fitting. The section where is enters the inlet fitting is quite small and looks like there is a packing plate held on with three or four screws. Maybe like a manual "thermostat" to restrict the water flow in really cold weather.
That fitting is for bringing outside water from the fire pump to the engine for added cooling during extended periods of pumping. That feature
is found on most pumping engines. It is the job
of the Engineer to regulate the flow and not blow up the cooling system, as well as the flow of water that is being pumped to the discharge
lines. I belive it is covered in title 22 of building fire apparatus, if I can remember back that far.