William Shatner returns to his native Canada, ironically to play an icon of United States literature.
Ian D. Clark (Clarence Eldridge) is originally from the same city in northern England as Thomas Craig (Brackenreid). They are both from Sheffield, South Yorkshire.
This episode takes place in 1903.
Mark Twain was famous for being born and dying in years when Halley's Comet was in the sky, 75 years apart. In 1903 he was 68 years old. William Shatner, who plays him, was at least 83 when this was filmed in 2014-15.