Before television there was radio, and before David Janssen as Richard Diamond, there was Dick Powell as Richard Diamond.
Richard Diamond, Private Detective was created by Blake Edwards and aired (in its radio version) from 1949 to 1953. Along with Dick Powell, the cast included Virginia Gregg as Diamond's girlfriend Helen, Ed Begley as Diamond's former police partner Walter Levinson, and Wilms Herbert as the comic relief cop Otis.
(The television show, 1957-1960, had Regis Toomey, Russ Conway, and Barbara Bain supporting Janssen. The television series had a slightly darker tone to it, but the television series also had Mary Tyler Moore's legs. Can it possibly be true that MTM's legs only appeared in seven episodes? Boggles the mind.)
"The Plaid Overcoat Case" was broadcast on NBC radio on December 28, 1951. You may recognize the voice of Sheldon Leonard as tough guy "Duke." This episode was written by Dick Carr and was directed by Nat Wolff.