The vast majority of the first series of The Avengers (1961) is sadly missing from TV archives and likely lost forever - only the first 15 minutes of this first episode and three complete later episodes, Girl on the Trapeze (1961) (which incidentally does not feature the character of Steed), The Frighteners (1961), and Tunnel of Fear (1961) (found in the hands of a private collector in 2016) have been recovered and preserved.
Harry Black was actually portrayed by an actor named Anthony Bate, who took Murray Hayne's place at the last minute - too late for the TV Times to change their cast listing. Similarly, Hazel Coppen stepped in at a late date, replacing Nancy Roberts as Madame Zenobia.
Puppy's final appearance: Juno was later killed in an accident on the London Underground on the way to tape a subsequent episode of The Avengers (1961) at ABC TV's Teddington studios. She would be replaced as Steed's pet dog by her offspring, Junia, in the Cathy Gale seasons.
Due to public interest in the episode's recovery, 'Tunnel of Fear' was released on Region 2 DVD on 9th April 2018.