Felicity referred to Oliver Queen of Earth X as "Oliver in the High Castle" a reference to the television series and novel, Man in the High Castle, which also depicts a world where the Nazis won World War II similarly, the television adaptation explored alternate dimensions in its second season.
For Colin Donnell to do his guest spot as the alternate version of Prometheus from Earth X, he wrapped up a full day of filming in Chicago Med, flew to Vancouver and shot his scene and then hopped back on a flight to Chicago to film Chicago Med again the next day.
The Earth-X version of Adolf Hitler died in 1994. As Hitler was born on April 20, 1889, he was either 104 or 105 years old at the time of his death.
SuperArrow was a joke ship created by Arrow's subreddit (r/Arrow) in an attempt to mess with the Olicity fandom, who a lot of fans despise. SuperArrow is a ship of Kara (Supergirl) and Oliver (Green Arrow). In this episode their Earth-X doppelgängers are revealed to be married, making this relationship canon in the Arrowverse.
Victor Garber (Martin Stein), Franz Drameh (Jefferson Jackson) and Dominic Purcell (Mick Rory) join Stephen Amell (Oliver Queen), Grant Gustin (Barry Allen), Melissa Benoist (Kara Danvers), Carlos Valdes (Cisco Ramon), Danielle Panabaker (Caitlin Snow), Caity Lotz (Sara Lance) and Emily Bett Rickards (Felicity Smoak) in appearing across all 4 Arrowverse shows.