Getting more outside air in the early truck and car cabs was usually done by a swing out windshield frame (in addition to a top cowl vent). With no insulation on the inner firewall, engine heat was always radiating into the cab interior.
During the final 10 years of this hinged swing out on Chevy and GMC, the windshield opened by a center mounted splined turn handle.
For the perfectionist these handles slightly changed during 1936-46. The earliest handle (1936-38) will not interchange with later due to the dash shape. When you want your restored truck just right, the following photos should be of help. NOTE: All are available from Jim Carter Truck Parts and are exact reproductions.
1936-38 Chrome Die-cast Knob
1939 Photo Soon with Black Plastic Knob
1940-46 Tan Plastic Knob