That was 'old school' manifold heater called a heat riser.
Open Position, exhaust goes straight out exhaust pipe.
Closed position, exhaust is diverted to heat floor of intake then out the exhaust pipe
There was a spring that would control the position of the flapper. Usually rusted away as in these photos.
When cold it would divert exhaust gases onto the floor of the intake manifold to keep it warm so gas would not condense out onto the surface of the cold manifold and pool on the floor.
These manifolds are usually bolted together and made of cast iron. They didn't realy flow very well.
Even my old '63 Chebbie had one of these. This knowledge has been around a long time.
Those jeeps with water heated manifolds used an electric manifold heater.
These would turn on when you had enough oil pressure and turned off when the intake was warm enough to keep the mixture from condensing out.
These manifolds are usually made of aluminimum.