During the early years of auto and truck design, most vehicles came with their windshields capable of tipping outward. This helped poorly insulated cabs to be more bearable during hot weather. Extra outside air would be forced into the cab and replaced some of the hot air radiating from the bare sheet metal firewall.
This idea was good but not without a few problems. Unfortunately, air movement depended on the speed of the vehicle. The faster the driving, the more air circulation. Too bad for the driver in stop and go city traffic during a hot summer day.
In GM trucks, water leaks into the cab developed as the rubber edge seal began to age. The under dash crank-out gear assembly (1936-46) was not easily reached and therefore almost never received lubrication. The gear would wear and later most became non-usable. It was then necessary to close the windshield frame permanently and the cab lost a major method of getting air flow on hot days.
The system was expensive to produce! A pair of swing hinges, a crank-out assembly, and two windshield halves added to production costs.
This windshield vent system was stopped with the introduction of the 1947 Advance Design cab. The two piece windshield now became permanently sealed. An insulated interior fire wall pad was standard. A left side cowl vent intake door forced outside air over the drivers feet and lower legs. (Of course, this was also when the truck was moving.)
A top cowl vent door (also on earlier trucks) now had a screen to prevent entry of insects. Thus, this was the end of the swing-out windshield. They were a great help on hot days when the vehicle was moving, but inevitable gear wear began a new set of problems for later years.