Don't forget the DH-4B which was a modified US built DH-4 for US Government use (aka Model 16 as produced/modified at Boeing). I am not sure if it was the first official US Post Office airplane used for Airmail service, but it was one of the earliest. They started operating in 1919, and eventually flew the cross-country routes - including through St. Joseph.
See "
DH-4 Mail-plane" for a recent restoration project of a DH-4.
View attachment 826462T