Here is the Chevrolet version of how the Air-Flow heater system works.
https://www.flickr.com/photos/advance-design_parts_co/17058542759/in/album-72157651718017649/
If the plate is in place any moisture drains out the weep holes (until it does not anymore due to plugging).
https://www.flickr.com/photos/advance-design_parts_co/15884899956/in/album-72157649468726416/
The above shows the louvre design intent from “The Cab That Breathes” circa 1947.
1947/48 were the only years that carried the Air-Flow (referred to in ‘47/48 as Fresh-Air Ventilator, Heater and Defroster) as a factory supplied option, RPO 385. The photo above is the same one that appears in both 47/48 Salesman Truck Data Books.
https://www.flickr.com/photos/advance-design_parts_co/48005151456/in/album-72157631868452754/
This photo is from ‘48 with the Chevrolet naming conventions.
https://www.flickr.com/photos/advance-design_parts_co/17037284497/in/album-72157651722976780/
This page from the FAM shows every hole and purpose for you.
https://www.flickr.com/photos/advance-design_parts_co/14237974312/in/album-72157631868452754/

1950 Chevrolet 3/4-Ton 3600
More pictures on Flickr