In the original draft of the script, the killer's name contained seven digits because Ellery deduced that the victim dialed his killer's name on the phone. But this was changed when script researchers realized that in 1947, only *six* digits were required for dialing a phone number and consequently not only was the killer's last name changed to accommodate so were the last names of two other characters exonerated.
Marcus Halliday refers to Howard Pratt as an "overage Caspar Milquetoast". Caspar Milquetoast was a comic-strip character published in two New York City newspapers from the 1920s to the early 1950s. The character was written as meek, bland, and perpetually nervous.
Joan Collins and Farley Granger acted together in Girl in the Red Velvet Swing
Kitty's address is given as 124 West End Avenue. No such address exists, but it would put her apartment somewhere in the mid-60s, near what later would be Lincoln Center.