The "war room" tracking the car for Scotland Yard is based on the RAF war rooms of WW2, right down to the cue sticks moving cars instead of airplanes.
With Poirot and Hasting's eclectic connections and client base, it is not implausible that they managed to somehow get hold of a early (work in progress) test version prototype of the "London" version of Monopoly, before a Patent was registered. Who better to test the board game concept than the legendary Hercule Poirot. However, the UK version of Monopoly was available in the U.K. after 1935. The game was loaned out to the London company John Waddington, Ltd., for review before the USA version was being sold in 1935. This would date the episode as taking place after 1935, however when a hotel book is signed in the episode the date is given as February 8, 1935.
During the police raid at the Chinese casino, one of the female extras with long hair, wearing a white dress is in several scenes.
She is walking up the stairs past Japp when he arrives, nine seconds later she is in the background having a drink, 15 seconds after that she is at Japp's side, and finally six seconds later she is across the room, moving away as Japp approaches the entrance to the opium den.
The actor that plays a policeman taking pictures from a surveillance car is uncredited, and also appears uncredited in at least two other Poirot episodes: in the King of Clubs he plays Mr Oglander and in The ABC Murders he plays a spectator at the horse race.
Barbara Barnes (Mrs Lester) appears later in series 8, Murder in Mesopotamia (2001), portraying Mrs. Leidner.