Files W.F.F. Archive Publications National Security Agency, Military Cryptanalysis, Part I, Course Lesson #8, 9, 10, 1941-1956
Military Cryptanalysis Part I: Monoalphabetic Substitution with Irregular-Length Cipher Units; Monome-Dinome Systems and Others
Military Cryptanalysis, Part 1, Course Lesson 10, Pars. 81-84; Miscellaneous Monoalphabetic Substitution Systems; Concealment Systems, William F. Friedman
Military Cryptanalysis, Part I, Syllabary Squares and Code Charts