The original screenplay had a German called Muller in place of the American Monroe, and the Rosas lived in Buenos Aires instead of Santiago - which makes sense, with the German presence in Argentina, and it being more noted than Chile for raising horsestock. Accordingly, Steed's journey as courier was Bogota-Santiago-Buenos Aires instead of Bogota-Lima-Santiago.
Gerald Harper would later star in "Adam Adamant Lives", the BBC's rival to The Avengers.