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Mine has an in-line valve in the heater hose. I's not original because it is plastic. So the answer to your question is, I don't know. I think they did have a brass in-line valve though. I may be wrong.
The fresh air heater like FortyNiner has in his truck would have a water valve that is controlled from inside the cab by a cable. My '49 truck has a three door heater so, the water must be turned on and off manually at the water valve on the right hand cylinder head. So, it would depend on the type heater you have as to which valve you would use.
Mark
Rezvani's Latest Post-Apocalytic Monster Is a Ford F-150 Raptor Underneath
Slideshow: Called the Fortress, the 850-horsepower pickup combines Raptor underpinnings with military-inspired features, survival equipment, and a starting price of $285,000.