This episode is based on "The Reigate Squires," a case that was known in America as "The Reigate Puzzle," and which featured a father and son called Cunningham who murdered their coachman, William Kirwan.
Richard Larke (un-credited) first appeared as "Sergeant" - unlisted on credits, it also fits that Sergeant does not receive a name. He appears twice: once telling Holmes (Ronald Howard) and Watson (H. Marion Crawford) that Lestrade instructed him to allow Sherlock's acceptance at Cunningham house; next, he met Lestrade outside Cunningham's and answered "no" to Lestrade's query as to Holmes' re-entry.
Unlike most Holmes stories, this episode and the next, "The Case of Lady Beryl", are solved within a twenty-four hour period.