[Lynn Eades]

You should be able to find a pressure valve. (try an add here on MLU?)
The part number is; 553 - Assembly, pressure valve radiator overflow.
As the thing indicates, it is what causes the pressure in the cooling system. They are adjusted (I think) to 3 1/2 psi (adjustable?) Some (later?) radiators are fitted with the correct neck for a 4 lb pressure cap, but yours looks to be the plain blanking cap.
Raising the pressure raises the boiling point of the water 4 degrees F. per pound of pressure, so a 3.5 pound cap raises the boiling point 14 degrees F. (at sea level) This is a good thing. In fact I'd say very important for a reliable carrier in the potential temps you might want to operate your carrier in.