They should have put two cables on the 53-55 Air Flow heaters.
One cable for Air Flow or Recirculating mode or both.
One cable for Defrost or no Defrost mode.
Reaching under the dash to turn on Defrost while driving is a hazard to your health.
It is, however, quite easy to understand how the 47-52 cable-control works - it simply provides varying amounts of defrost to the windshield and/or heat directed towards the floor. The outside, fresh air, is always going through the heater core.
I think one of the reason why they changed the design of the 47-52 air flow heater.
And put a door on where the outside air comes in.
It was acting like a vent.
In the summertime air could be push threw the louvers (while driving) across the heater core, unnecessarily heating the cab.
Door, No air flow.
