1. Alexander, C. H. O. 1944. Stecker knock-out. Addendum to captain Walter J. Fried’s report No. 8 of 23 March 1944. Record Group 457 of the Historic Cryptographic Collection, File Unit Nr. 2612, NARA, College Park, MD. http://research.archives.gov/id/2809159. Transcribed by Frode Weierud: http://cryptocellar.org/Alexander/sko.pdf (accessed February 28, 2015).
2. Alexander, C. H. O. 1945. Cryptographic history of work on German naval Enigma. Catalogue reference HW 25/1. The National Archives, UK. Transcribed by Graham Ellsbury: http://ellsbury.com/gne/gne-000.htm (accessed January 26, 2015).
3. Dilly Knox—A Reminiscence of this Pioneer Enigma Cryptanalyst
4. THE TURING BOMBE: WAS IT ENOUGH?
5. Enigma General Procedure. 1940. RG 457, HCC, Nr. 1679. http://research.archives.gov/description/2808122. Transcribed by the late Tony Sale: http://codesandciphers.org.uk/documents/egenproc/eniggnix.htm (accessed April 30, 2015).