US Army Technical Manual TM 11-459 Instructions for Learning International Morse Characters 1943-06-02 "This manual is provided as a text for students learning and acquiring skill in the use of International Morse characters. It may be used by students being taught the characters by either of two approved teaching methods. The tape method makes use of a recorded tape with associated equipment as a source of character sounds. The record method employs phonograph records and reproducing equipment as a source of character sounds." There's also a set of MP3s of the instructional phonograph records at Archive.org: U.S. Armed Forces Institute Basic Radio Code (1942) dit ditCover title"June 2, 1943"--CoverIncludes index