The "Telephone Showdown" scene between Inspector Brackenreid and Constable Higgins is shot in the style of Sergio Leone: Mexican standoff, closeups that become more and more extreme, concerned onlookers, plus the music is reminiscent of an Ennio Morricone score.
The "Oak Park Church" that Frank Lloyd Wright (1867-1959) refers to is the Unity Temple in Oak Park, Illinois. It was built between 1905 and 1908 with a budget of $45,000 ($1.25 million in 2019). Due to the small budget, Wright constructed the church out of concrete. It has been a historic landmark since 1971; in 1991, it was chosen as #6 on the list of the 100 most significant buildings in the United States in the previous 100 years; it was also chosen as one of 17 buildings by Wright that should be retained as his architectural contribution to American culture. The concrete has suffered from extensive water damage, and a $23 million restoration was begun in 2015.
Actually, there are NO structures in Toronto by Frank Lloyd Wright.
Sophie McShera is the fourth Downton Abbey (2010) alumnus to appear as a guest star. Thomas Howes as Winston Churchill in Winston's Lost Night (2013), Brendan Coyle as Rankin in A Merry Murdoch Christmas (2015), and Samantha Bond as Lady Suzanne Atherly in Great Balls of Fire: Part 1 (2016) & Great Balls of Fire: Part 2 (2016).
Ether, the anesthetic used in this episode, was discovered in 1540 by Valerius Cordus, a Prussian botanist, though it was not used as such until the 19th century.