When that flap is open the inner flap is closed and cab air recirculates. When that flap is closed the inner flap is open to allow outside air into the cab.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.