Wikipedia
MIS-X was a section of the United States Department of War that operated during World War II. It aided U.S. servicemen held as prisoners of war and those evading capture in enemy territory. The section, which was modeled after the British MI9, was disbanded at the war's end.
MIS-X was based in Fort Hunt, Virginia. Secret equipment such as small compasses, maps, and radios were smuggled into German prisoner of war camps; to avoid compromising legitimate aid organizations like the Red Cross, MIS-X invented its own aid organizations and created aid packages from these, in which secret items were hidden. A radio code was devised to send messages via BBC broadcasts; this was based on Morse code: a one-syllable word was a dot, a two syllable word a dash. Such messages were preceded by a bell tone. A method of hiding a message in innocuous-looking outgoing letters to the United States was devised: all incoming letters were screened for such messages before delivery. These were signaled by having the date in numbers rather than letters. American officers who were privy to the secret and codes debriefed incoming prisoners and sent any useful information back to the United States.