The episode's full title was "The Mystery of the Second Stain" - the original story being "The Adventure of the Second Stain." Here, Holmes scores a diplomatic coup when he stops a certain letter from falling into the wrong hands.
The 16 episodes of Sherlock Holmes starring Gielgud and Richardson were produced by Harry Alan Towers (1920-2009) for his "Towers of London" company. They were first broadcast on the old BBC Light Programme in 1954, and by NBC in 1955; they are still regarded by many as the definitive Holmes series. Heritage Media acquired exclusive distribution rights to the series from Towers in 1994; the series was licensed for several years with great success to Hodder Headline and reverted to Heritage at the end of 2009.
The scripts were written by John Keir Cross, closely following the original stories of Sir Arthur Conan Doyle, but renaming some of them. The music for the productions was composed by Sydney Torch, with the famous violin cadenzas played by Alfredo Campoli. Other actors appearing included Robert Rietti, and John Gielgud's brother Val (later Director of Drama at the BBC), who played Mycroft Holmes. Orson Welles took the part of Professor Moriarty. Quite possibly other "big names" on the BBC roster appeared in the series anonymously too.
The recordings were carefully remastered so as not to detract from the ambience of the original performances.