Take the mailbus!
Another KSHS picture. Description:
F.J. Henderson had a contract to deliver the United States mail on the Syracuse-Johnson-Elkhart mail route from 1913-1920. In this photograph, Henderson stands next to the model "T" Ford he used to deliver the mail in 1916.
Mr Henderson's T is a wonderful mix of pieces, nicely carpentered together to carry both mail and passengers.
The 'bed' is probably homemade. The passenger part is clearly from a center-door sedan:
Well, no. I can't find any centerdoor from any brand that has the same shape and structure. Also, the centerdoor sedan was a new (and bad) idea in 1916 and probably wouldn't be found in a junkyard.
The cab is equally unpinnable. Truck cabs were generally aftermarket. Many cabs had an oval 'opera window', but I can't find an exact equivalent online or in books. Here's a 1917 panel truck, more typical of mail trucks. Short oval.
And here's a screencap from a Lawless Years episode. Long oval but entirely different door.
Best guess, then: The whole thing was Henderson's design, using pieces from Ts and houses and horse-powered stagecoaches to make a unique mailbus.
Obviously you couldn't carry passengers in a mailtruck now. Regulations and liability.
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Later note: KSHS
also has a picture related to an earlier version of the same mail route...
This dugout was located on the homestead of Luther Kreigh, on the west side of present-day Highway 27, seven miles north of Johnson City, Stanton County, Kansas. Luther Kreigh ran the stage line and mail hack between Syracuse and Richfield and this dugout served as a stage stop on that line. The stage changed teams here and a post office located in the building was called on the maps "Liverpool."
Note 'stage route and mail hack'. So Mr Henderson was following a stagecoach tradition. Was he using the same stagecoach as the back part of his 'mail hack'?