This episode was co-written by Don Adams with his sister Gloria Burton.
The plot of a commoner impersonating a nobleman to whom he bears an uncanny resemblance was first popularized in Anothony Hope's novel "The Prisoner of Zenda". The novel was adapted into a celebrated movie in 1937 starring Ronald Colman as the identical king and commoner. Since Don Adams has said that his famous Maxwell Smart voice developed out of a Ronald Colman impression, a take-off of the movie must have seemed almost inevitable.
This episode was co-written by Don Adams.
Max: Ah, yes. I remember it well.
Max and Princess Marta trade conflicting memories of a date reminiscent of song I Remember It Well from the musical Gigi (1958).
While being quizzed on his knowledge of King Charles' family, Max says his mother was Queen Helena of Ruritania. Ruritania is the fictional kingdom which was the setting for The Prisoner of Zenda.