John Herivel (English Wikipedia)

"Bletchley Park – Remembering Herivel and the Herivel Tip". 23 January 2011. Retrieved 5 August 2016. However, on the 1st of May, the Germans changed their methods, rendering the existing techniques inoperable. Alan Turing and his team had already anticipated this change, and were building a machine (the Bombe – in effect a computer) to decode the messages. That left a people [sic] from the 1st of May to the 1st of August, while the Bomb was being built.

bletchleypark.org.uk

Hinsley et al. 1988 says the first British bombe arrived 18 March 1940. Hinsley states that "Welchman's ... explanation of the 'Herivel tip' is incomplete." F. H. Hinsley, E. E. Thomas, C. A. G. Simkins, C. F. G. Ransom, British Intelligence in the Second World War: Its Influence on Strategy and Operations, Vol 3 Part 2, Cambridge University Press, British Crown Copyright, ISBN 0-521-351960 p. 954

Rijmenants, Dirk, "Enigma Message Procedures", Cipher Machines and Cryptology, retrieved 19 November 2009

Sullivan, Geoff; Weierud, Frode (2005), "Breaking German Army Ciphers" (PDF), Cryptologia, vol. 29, no. 3, pp. 193–232, doi:10.1080/01611190508951299, S2CID 23474156, archived from the original (PDF) on 24 August 2006, retrieved 20 July 2011

"Obituary – John Herivel", Methodist College Belfast, 28 January 2011, archived from the original on 27 July 2011, retrieved 20 July 2011

Sullivan, Geoff; Weierud, Frode (2005), "Breaking German Army Ciphers" (PDF), Cryptologia, vol. 29, no. 3, pp. 193–232, doi:10.1080/01611190508951299, S2CID 23474156, archived from the original (PDF) on 24 August 2006, retrieved 20 July 2011

Sullivan, Geoff; Weierud, Frode (2005), "Breaking German Army Ciphers" (PDF), Cryptologia, vol. 29, no. 3, pp. 193–232, doi:10.1080/01611190508951299, S2CID 23474156, archived from the original (PDF) on 24 August 2006, retrieved 20 July 2011

"Special Forces Obituaries: John Herivel", The Telegraph, 20 July 2011, retrieved 20 July 2011

"Obituary – John Herivel", Methodist College Belfast, 28 January 2011, archived from the original on 27 July 2011, retrieved 20 July 2011
Sullivan, Geoff; Weierud, Frode (2005), "Breaking German Army Ciphers" (PDF), Cryptologia, vol. 29, no. 3, pp. 193–232, doi:10.1080/01611190508951299, S2CID 23474156, archived from the original (PDF) on 24 August 2006, retrieved 20 July 2011