The opening music and titles to Poirot together set the scene that these are tales from the 1930's. Towards the end of the titles, a stylised and complete Battersea Power Station is shown, which was built in two phases the second of which was not finished until the early 1950's.
The metric system was not adopted in England until 1962 and yet it is used here in the 1930s.
Poirot shouldn't just touch the brim of his hat when saluting ladies; hat should be removed.
In several episodes, Poirot says "Viens!" ("Come!") to a character. Poirot would never use that familiar form of the verb "venir" to someone he hardly knows. He would use the more formal "Venez!".