That control valve is pretty sophisticated for a truck heater from the late 40's & early 50's.
The capillary tube mounts near where the hot air comes out of the heater.
It "reads" the outgoing air temperature &
it actually "controls" the valve by opening & closing the valve automatically.
If you were to set the warmer knob on medium, the valve would maintain that setting, without you havin' to make any more adjustments.
