As I was writing a chapter about the uniforms of government workers for my book, On the Job: A History of American Work Uniforms, I found this stereograph card at the Library of Congress (you can access the metadata by clicking on the image below):
This image was made at the 1904 World's Fair in St. Louis, Missouri. It shows hundreds of urban postal carriers in their identical uniforms (pants, button-down shirts, neckties, and hats) marching in military formation on the "Pike," an avenue full of spectacles from around the world. (In my book, I describe how this demonstrates the impact of military uniforms, equipment, and practices on other branches of the federal government).
The reason there are two nearly-identical images is because this is a "stereograph".card. The first stereographs were exhibited in 1851 at the Crystal Palace exposition in London. Popular among middle-class families, stereograph cards were purchased in sets as souvenirs and educational tools in order to show viewers the wonders of the world in 3D (long before IMAX and VR). They peak of their popularity was from the 1870s to the 1920s.
So why photograph postal carriers? What was the attraction?
Among the many displays at the 1904 World's Fair was a working post office and exhibit of archival materials from the US Postal Department. As described in an illustrated book about the fair's attractions (which was also prepared as a souvenir):
EXHIBIT OF POSTOFFICE DEPARTMENT
Across the main aisle was see the postoffice display, including a working mail car with the clerks tossing the letters into the proper pigeon hole or pouch with the rapidity that prevails on a car rolling 70 miles an hour over the railroads. From the burro that collects the mail in the mountains to the white trolley-car familiar to city dwellers, every link in the postoffice chain was complete in the exhibit.
A valuable collection of old-time relics from the postoffice museum at Washington illustrated the crude beginnings of the postal system. One of these relics was an old-fashioned stage coach that formerly carried United States mails through a portion of the Louisiana Purchase territory. President Roosevelt, upon seeing it first, examined with a soldier's interest the bullet holes which stage robbery and Indians shot through its leather curtained sides. Generals Sherman and Sheridan and President Garfield rode in this old stage-coach in their strenuous days of frontier life. Even the type of "mail wagon" used in Alaska sledges, pulled by dogs over the frozen snow, were shown in their collection.
DEAD-LETTER OFFICE EXHIBIT
Another interesting feature was the display from the Dead-Letter Office museum, showing stray, tabooed articles found in the mails, ranging from infernal machines to living serpents, several of the latter being rattle-snakes concealed in innocent appearing packages, calculated to around no suspicion in the mind of the jeopardized recipient. Severe punishment is the penalty provided for such offenses.
COMPLETE POSTOFFICE IN OPERATION
In connection with this department there was a complete postoffice in operation in the building for the accommodation of the thousands of the world's fair officials, attaches and employees. There was also a special issue of world's fair stamps. The issue consisted of 90,000,000 of the 1-cent variety, 225,000,000 2s, 7,500,000 3s 9,500,000 5s and 6,500,000 10s. The designs were all commemorative of the Louisiana Purchase and were more beautiful than any special stamp ever issued by the Stamp Division. The postoffice had every facility for the transaction of money-order and registry business, the sale of postal supplies and the receipt and dispatch of mails.
J.W. Hanson, The Official History of the Fair, St. Louis, 1904: The Sights and Scenes of the Louisiana Exposition: A Complete Description of the Magnificent Palaces, Marvelous Treasures and Scenic Beauties of the Crowning Wonder of the Age (St. Louis,1904): 461-462. Transcribed from a copy held by the Lilly Library at Indiana University.