The last episode where anyone addresses Smith as "Colonel."
Although cut from the final version, there was a scene where the Robot identifies Debbie (the Bloop) as being female, which would have clarified the gender question with the chimp throughout the series.
Debbie the Bloop (a dressed up chimp) makes her debut.
Dr. Smith claims to have batophobia, an incurable fear of heights. Sort of, almost. Batophobia is the fear of tall buildings, an oddly specific subset of acrophobia, the fear of heights.
The title may be based on the movie Island in the Sky (1953), starring John Wayne. There may be other sources, including a 1952 science fiction novel by Arthur C. Clarke.