"Final Protocol: Article IV" from "Appendix B. Final Protocol, First International Radio Telegraphic Conference, Berlin, 1903" in History of Communications-Electronics in the United States by Linwood S. Howeth, 1963, page 547.
"Distress Signalling" by G. E. Turnbull, The Year-book of Wireless Telegraphy and Telephony, 1913 edition, pages 318-322.
"Codes", Manual of Wireless Telegraphy for the Use of Naval Electricians (first edition) by S. S. Robison, 1906, page 112.