The idea for the play Poirot and Hastings watch was later used by Christie in her novel "Murder in Three Acts."
Agatha Christie's The Third Floor Flat is from the September 1974 published collection of short stories called "Poirot's Early Cases", which charts some of the cases from Hercule Poirot's early career, before he was internationally renowned as a detective.
The Third-Floor Flat was first published in Hutchinson's Story Magazine, January 1929. It was republished in the collection Three Blind Mice and Other Stories in 1950.
The lift used, is in fact the main lift located in the tower of the Salvation Army's UK Training College in South London.
This episode takes place in October 1935.