Dr Watson says he practised medicine in India attached to the 5th Northumberland Fusiliers. The Northumberland Fusiliers was formed as the county regiment of Northumberland on 1st July 1881. They fought in the Second Boer War but never in India.
Jeremy Brett's former father-in-law Raymond Massey previously played Sherlock Holmes in another adaptation of the same short story: The Speckled Band (1931).
Jeremy Kemp (Dr Grimesby Roylott) previously played Baron Karl von Leinsdorf, a villain opposing Sherlock Holmes, in The Seven-Per-Cent Solution (1976).
Two of the actors in this episode both appeared as characters in episodes of Star Trek The Next Generation. Namely, Jeremy Kemp (step-father) who played Captain Jean-Luc Picard's older brother (Robert) in the episode "Family" in 1990 and Rosalyn Landor (daughter) who played Brenna Odell in the episode "Up the long ladder" in 1989.