The International Harvester "All-Fresh-Air" Heater
(Source: International Light Duty Trucks, International Harvester Co., Publication CR-485-C)
The International "All-fresh air" heating, ventilating and defrosting system, supplied as optional equipment on all R-line models, provides all-weather comfort on all R-Line models, provides all-weather comfort the year 'round. Fresh-air intake, through the cowl ventilator, eliminates the possibility that foul air will be taken into the system. In R-Line Internationals equipped with this system, drivers choose their own weather - comfortably warm in winter, refreshingly cool in summer. Ask your International Truck Dealer or Branch for more information on the advantages of this heating, ventilating and defrosting system which provides year'round comfort, protects the driver's health, and contributes to safe driving.
In case it might help others, I've taken a few photos of the heater installation in Cornelia so that you can see the wiring and the routing of the ducting. You can click on any of the photos to see a larger image and much more detail: