About half way down page 2 of this thread, you ask about this censor marking:
You suggest that it is not British, however in my limited references to WWII-era censorship, the only reference I can find right now is the scan above in the British section in this book:
Between pages 83 and 91, the authors note that "On August 28, 1939 censorship regulations under the Emergency Powers Act were published. Outward-bound mail, mail from protected areas of the I.K., incoming mail, mail brought into the country by travelers and business mail were subject to examination .... The censorship office in London censored all airmail and surface mail to and from neutral countries in Europe and the rest of the world with the exception of the mail to and from the United States of America, China and Japan which was examined at Liverpool."
A calendar of events on page 83 notes that the war between Germany and the UK breaks out on Sept 3, 1939, and the US protested against thee examination of neutral mail on Dec 27, 1939.
Two variants of the boxed mail exist, this one with "Censored" text not as wide as the text above it (for commercial mail), and a second type introduced later with "censored" equal in width to the text above (for philatelic mail). Examples of both box types are currently on eBay
including this one - of the type matching yours, with a pencil notation that it is British censorship.
Here is another piece which recently sold on eBay
for $45. The description reads, in part: "The postcard is dated August 16, 1939, and it was written on board the ship near Colon in the U.S. Canal Zone and sent to Hamburg, Germany. It is postmarked with a cancellation clearly reading Aug. 21, 1939 at what appears to be 6 p.m. and is franked using one 3c (Scott CZ 122) stamp. It also has a boxed red "Released by Censor" cancel which was applied in the U.K. (H3B), which is scarce with little use."
I suspect others with more military postal history experience will be able to build on this - especially if they have the book which catalogs the mark as "H3B".