The earlier models of that style heater were call "fresh air" because outside air was moved through the heater at all times.
The 53-55 models of that heater had the door/flap on the front for recirculating interior air and were called
"air flow". That flap/door operates as Cletis describes in the previous post. Look at the illustration and you can see that it shows a partially open flap/door (recirculating a little inside air and letting in a little outside air).
Several years ago, the "new owners" of
GSPP - Golden State Pickup Parts kindly found me a correct length/knob 54/55 control cable for this style heater (a reproduction cable/knob that they might have made for me?).
