- ^ Doyle, Arthur Conan. "Chapter 2: The Statement of the Case". The Sign of the Four. He disappeared upon the 3d of December, 1878,—nearly ten years ago. & About six years ago—to be exact, upon the 4th of May, 1882
- ^ a b c Redmond, Christopher (2009). Sherlock Holmes Handbook: Second Edition. Dundurn Press. pp. 14–15. ISBN 9781459718982.
- ^ Smith, Daniel (2011). The Sherlock Holmes Companion. Castle Books. p. 30. ISBN 978-0-7858-2784-9.
- ^ Honce, Charles (1944), "Sherlock Holmes in the News", in Smith, Edgar W. (ed.), Profile by Gaslight, Simon and Schuster, pp. 74–75
- ^ Green, Richard Lancelyn (ed.). The Adventures of Sherlock Holmes (Oxford University Press, 1998). p. 11. ISBN 978-0191605598
- ^ Klinger, Leslie (ed.). The New Annotated Sherlock Holmes, Volume III (New York: W. W. Norton, 2006). p. 214, 269. ISBN 978-0393058000
- ^ Dickerson, Ian (2019). Sherlock Holmes and His Adventures on American Radio. BearManor Media. p. 6. ISBN 978-1629335087.
- ^ Dickerson, Ian (2019). Sherlock Holmes and His Adventures on American Radio. BearManor Media. p. 49. ISBN 978-1629335087.
- ^ De Waal, Ronald Burt (1974). The World Bibliography of Sherlock Holmes. Bramhall House. p. 384. ISBN 0-517-217597.
- ^ De Waal, Ronald Burt (1974). The World Bibliography of Sherlock Holmes. Bramhall House. p. 390. ISBN 0-517-217597.
- ^ Payton, Gordon; Grams, Martin Jr. (2015) [1999]. The CBS Radio Mystery Theater: An Episode Guide and Handbook to Nine Years of Broadcasting, 1974-1982 (Reprinted ed.). McFarland. p. 196. ISBN 9780786492282.
- ^ Bert Coules. "The Sign of the Four". The BBC complete audio Sherlock Holmes. Retrieved 12 December 2016.
- ^ Wright, Stewart (30 April 2019). "The Classic Adventures of Sherlock Holmes: Broadcast Log" (PDF). Old-Time Radio. Retrieved 1 May 2020.
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