Steve, I am old enough to remember people, including my father, using cardboard to restrict the airflow through the radiator of his trucks, etc. As I try to reconstruct the need for this, methyl alcohol was used to lower the freezing point of the water in the cooling system. Glycol anitfreeze was not available at that time (immediately after WWII). When travelling at any significant rate of speed (over 30 mph) in cold weather the alcohol could freeze in the radiator from the wind chill. The engines were relatively low compression and did not run very hot. As a result there were times when the air flow through the radiator could cool the coolant to the point that it would freeze before it got to the bottom of the radiator. The point of the cardboard was to reduce the airflow through the radiator to prevent freezing. I can remember draining the radiators at the end of the day (and refilling them the next morning) so that they wouldn't freeze overnight. It was quite an art to figure out how much of the radiator to cover. I can remember having both extremes happen - too much cover and the rad boiled over and not enough cover and the coolant froze. Boiling over was the easier situation to deal with. When we switched to trucks with diesel engines, they ran 24/7 through the winter.