The National Postal Museum's Pushing The Envelope blog posts, "This advertisement, for the White Motor Company, was published in a number of magazines, including the May 24, 1941 issue of 'The Saturday Evening Post.' The company touted their longevity, 'for 40 years the greatest name in trucks' appears at the bottom of the page, and used their connection with the U.S. Post Office Department to promote itself. The bus that appears in the advertisement was used in the Highway Post Office (HPO) bus service. As railroad passenger traffic declined, railway companies were pulling more and more trains out of service. As a result, the Department began to outfit buses with mail sorting equipment for use on lines as Railway Mail Service to some towns was cut."
It goes on to say, "The advertisement refers to the first HPO bus, which was put into service that year. That bus, manufactured by the White Motor Company, is in the collection of the National Postal Museum. The ad references America’s most famous mail moving service, the Pony Express, at the top of the image. Although the Pony Express was not operated by the U.S. Post Office Department, it has become connected to the system in popular memory. The ad also includes a map of the first HPO route, which operated between Washington, DC, and Harrisonburg, Virginia."
For more on the Highway Post Office Bus, click here.