Enclosed Mail Trucks
In 1921, the Post Office Department began to regulate its fleet. They focused on three car companies for the design of their vehicles: Ford Motor Company, Commerce, and White Motor Company. Ford trucks eventually became the dominant vehicle in the fleet. Trucks with enclosed interiors (different from the screened interiors) were produced at this time, generally at a later date than the screen trucks. These trucks were in use through the 1920s and 1930s and even into the 1940s, although during the Depression and WWII years, new vehicles were not produced. Thus, wear and tear is visible as the trucks become older.
The 1931 Ford Model A mail truck, seen in the image below, was popular for mail delivery during the 1930s and into the 1940s.

Ford Model A Mail Truck: Image courtesy of the Smithsonian National Postal Museum online exhibit
The truck in the image below was probably also produced in the 1930s. It is larger than the 1931 Model A, has now windows on the rear portion, and has sturdier wheels.

Enclosed Mail Truck: Image courtesy of the Smithsonian National Postal Museum online exhibit







