According to news items, Lionel Barrymore was originally cast in the role of the codger "Thevenet" and Marlene Dietrich as the housekeeper, but Barrymore was sick and Dietrich wasn't interested. A MGM "Final Cast" list, dated Jun 1951, includes Mercedes McCambridge as "bit girl," but she was not in the film and the inclusion may have been made in error.
On the reverse side of the paper from the IOU, it reads the beginning draft of a famous prose poem written by Edgar Allan Poe:
"Annabel Lee [title]
It was many and many a year ago
That In the kingdom by the sea
a maiden there lived whom you may know
By the name of Annabel Lee; "
During the film, "Dupin" recites the second verse of Edgar Allan Poe's 1845 poem "The Raven." Contrary to the implication in the film that no one would have known Edgar Allan Poe's name in 1848, at that time he was already a highly regarded poet. Poe died in 1849 at the age of 40.