I bought this cover for the War Department marking. The USAT Republic would no doubt stop in the Canal Zone on her regular route to refuel, where this letter entered the mail.
The USAT Republic was initially commissioned in 1917 as the USS President Grant. As such she served as a troop carrier during World War I. In 1931 she was transferred to the army, and after retrofitting was renamed the USAT Republic. After her refit, she sailed to various pacific ports including San Francisco, Honolulu and Manila. In 1932, her mission was narrowed to a route between New York and Honolulu (passing through the Panama Canal each time).
In 1941 she was returned to the US Navy, recommissioned as the USS Republic. She sailed mostly in the Pacific serving as a supply ship during World War II. After the war, in 1946, she resumed service to the Army again as USAT Republic for a few years before she was decommissioned and transferred to the US Maritime Commission where she would serve as part of the reserve fleet. She was eventually sold for scrap in 1952.