Holmes finds a picture of a South American landscape on a mantlepiece and says "See those unscalable cliffs which time and the foot of man have never touched, where monsters from the dawn of history might still roam". This is a reference to The Lost World, Holmes creator Arthur Conan Doyle's 1912 adventure novel, set in an unexplored plateau hidden in the Amazon jungle where dinosaurs still live.
Daniel Massey, who portrayed J. Neil Gibson, was the former brother-in-law of Jeremy Brett, who was married to his sister, actress Anna Massey. Also, this was the only time Daniel Massey and Edward Hardwicke appeared together, although they had been friends since childhood. Their fathers, Raymond Massey and Cedric Hardwicke appeared together in Things to Come (1936).
Sherlock Holmes is right-handed, and since Jeremy Brett was left-handed, a right hand double was used for writing or drawing scenes.
However, during the archery scene in Thor Bridge, Sherlock Holmes uses his left hand to shoot arrows.
Daniel Massey (J. Neil Gibson) was the son of Raymond Massey, who played Holmes in The Speckled Band (1931).
The setting for Senator Gibson's residence was Capesthorne Hall, a country house near the village of Siddington, Cheshire, England.