The painter of the dead wife's portrait is the same artist who painted the portraits seen on the staircase in the previous "Behind the Locked Door"
Telly Savalas and crew drive a 1955 Oldsmobile 88 4-door sedan.
The other car in the garage next to the Rolls Royce is a Jaguar XKE.
In order to prevent an accidental use of a real address, the zip code on the envelope is 735462 (six digits). ZIP codes were only then being introduced.
While the zip-code on the envelope being addressed by Mr. Hitchcock is fake, the street address, 3900 Lankershim Blvd is real, being what is now building 2160, on James Stewart Ave inside the Universal Studios lot.