'The Mysterious Affair at Styles' was Agatha Christie's first novel, published in 1920, and also her first to feature the character of Hercule Poirot.
The character Cynthia is loosely based on Agatha Christie herself and her family hosted a Belgian refuge.
The location used to represent Styles Court in "The Mysterious Affair at Styles" (Chavenage House) is different to the location used in Curtain: Poirot's Last Case (2013) (Shirburn Castle), when Poirot and Hastings return to the site of their first adventure.
The red-white-blue ribbon worn by Hastings is the 1914 Star; the bronze clasp surmounted indicates the wearer was in actual combat. Additionally, Hastings is referred to as "Lieutenant Hastings" near the beginning of the episode, and the markings on his uniform sleeve correspond with this rank. At some later point, he earned the title of Captain, which is how he is addressed in most episodes.
Original literary source : "The Mysterious Affair at Styles", novel by Agatha Christie, John Lance Co., New York, October 1920.