"U.S. TRANSPACIFIC SEA POST"
A cover posted and postmarked aboard ship at an official post office of the "U.S. SEA POST SERVICE".
This postmark is from the second sea post line on the Pacific, from San Francisco via Honolulu, Kobe, Shanghai, Hong Kong to Manila.
These postmarks can be identified by the letters "SF" in the CDS. They were used only in 1924 and 1925, beginning on 05 Jan 1924.
They had numbers from 15 to 23, each of which was permanently assigned to a ship.
As the indicator "WEST" says, cancelled on the voyage from San Francisco to Manila - on 26. July 1924 - postmark with the number "17".
Addressed to "Arc-en-Ballois", France. On the reverse the arrival postmark from there dated 18. August 1924.
An envelope like those freely displayed in 1st class cabins on many shipping lines. The ship name,
here "PRESIDENT POLK", does not guarantee that this letter was postmarked on this ship. From the "PRESIDENT POLK"
this is my second letter. Both with different numbers.
Much about this stamp has not yet been definitively clarified and is subject to its own research.
"Seaposts of the USA", Roger Hosking
"United States Sea Post Cancellations - Part 2 - Trans-Pacific, Central & South American Routes", Booklet No. 55, Philip Cockrill