that is the fresh air or deluxe heater. There are deluxe heater controls, (shown here) https://www.americanclassic.com/1955-59-Deluxe-Heater-Control-Assembly-w_-switch-Chevrolet/productinfo/68%2D478/
There are cables that go from the heater control to the heater itself. That valve controls the hot water flow through the heater core (as well as onto the floor in a lot of cases!)
