Two completely different designs, recirculating has a round core and just shuts on/off by hand for heat or defrost. It pulls air from inside the cab thus the recirculating name. The Air-Flow has a temperature control valve (varied thoughts on effectiveness) for heat control adjustment. It has a square core and pulls air from outside the truck through the passenger side louvers. It has a dash mounted cable to direct the heat/defrost function. The model with the door to dump heat right in the cab only appeared in late 1953 and carried on to 54/55.
http://www.flickr.com/photos/advance-design_parts_co/8129369078/in/set-72157631868452754
http://www.flickr.com/photos/advance-design_parts_co/8129369936/in/set-72157631868452754


1950 Chevrolet 3/4-Ton 3600
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