Thanks everyone!
The reason I asked if anyone thought if the St. Louis hotel cover was "philatelic" mail was to see if they recognized the addressee, Hiram E. Deats of Flemington, NJ.
Hiram Deats is in the
APS Hall of Fame. He was the President of the APS when this cover was mailed. Here are some excerpts from his biography at the APS website:

Hiram Edmund Deats
(May 20, 1870 – March 16, 1963) Flemington, New Jersey
Deats was one of the foremost U.S. collectors of the 19th century. Beginning as a youngster, he built the best collections of U.S. and Confederate States stamps of his time. Deats was member No. 36 of the American Philatelic Association (now the APS), joining in 1886 at the age of 16. For the next two decades he served the Association in various capacities, including president (1904-1905).
He formed an enormous philatelic library, a close second to that formed by John K. Tiffany. In 1952 Deats dispersed his library and duplicates. The major portion went to the Free Library of Philadelphia where much of it still resides. The enormous quantity of duplicates, plus material removed from the Free Library, came on the market over the next two decades in a deluge of mostly 19th century literature unprecedented in philatelic history.
Deats amassed an unsurpassed collection of U.S. revenues. In 1888 he acquired the collection formed by E.B. Sterling. His fabulous revenue collection was used to write An Historical Reference List of the Revenue Stamps of the United States Including the Private Die Proprietary Stamps (1899) (With co-authors George L. Toppan and Alexander Holland.) The book was reprinted in 1979 under its familiar title: The Boston Revenue Book.
Deats signed the Roll of Distinguished Philatelists in 1933.

In 1935, Hiram Deats, an avid Confederate collector, meets August Dietz, Sr., founder of the Confederate Stamp Alliance.