This is the first of two appearances Tim Matheson makes in the series, both times in the role of Beaver's friend Mike Harmon. His second appearance comes later in Season 6, Episode 38, "The Clothing Drive."
Mike likens the name 'Prisoner' on Beaver's letter to a comic book story titled 'The Count of Monte Cristo,' which would be an adaptation to the novel Alexandre Dumas first published in 1844 about a man unfairly imprisoned.