I discovered a delightfully Columboesque radio show from 1946, "Murder For Mathematics":
http://www.dieselpunkindustries.com/radio_shows/molle_mystery_theater/Molle%201946-01-25%20Burn,%20Witch,%20Burn%20(AFRS%20MystPhs%20#166).mp3

It is remarkably similar to "Prescription Murder" and has several later Columbo cliches. There are many differences, but I think Columbo fans will enjoy the similarities.

Similarities:
-The murderer has a somewhat sympathetic motive - he's in debt to a crook.
-The audience witnesses a man murdering his ex-wife.
-The police assume it's not a murder, but bring in Inspector Evans "anyway".
-Evans makes his first appearance late and disheveled - seven minutes into a 23 minute show.
-Evans appears to be from a working class background, with a Bronx accent.
-He's not well spoken, with "um" and "eh" verbal crutches.
-Evans is squeamish when he views the corpse. "Gosh."
-The police at the scene present their theory. Evans shrugs it off with "Maybe".
-Evans suspects the real murderer immediately, but plays dumb.
-Evans doesn't look like a policeman. He has to show his badge, twice.
-He finds trivial faults in the suspect's testimony. He calls them "wrong answers."
-He calms the suspect's accomplice: "He's not in any trouble. Just a checkup."
-Evans poses as a bumbler. "I usually forget things...I'm not very good that way."
-He plants seeds of distrust in the mind of the accomplice. "You know the other girl he had.."
-He asks his suspect to help solve the case, pretending the suspect is no longer a suspect.
-As in "Prescription Murder", he presents the suspect with a red herring "real killer".

Differences:
-Evans is more gruff and aggressive.
-There is no "bumbling genius" humor as in later Columbo episodes.
-Evans does not force the suspect to pretend to be charmed by him.
-There is no cat-and mouse dynamic: the unspoken "I know that you know that I know that you are the murderer".

Hello, the file can't be downloaded, try some other online file space...thks

Sorry, I got the title wrong. It's called Mathematics Of Murder, an episode of the Molle Mystery Theater.

Here's are two links to the episode on Youtube, both incorrectly labelled "Mathematics For Murder":
https://www.youtube.com/watch?v=CuQ_uPB7ybA
https://www.youtube.com/watch?v=n12cViYkFcM

I also nominate the movie Dial M for Murder as another possible Columbo prototype. The audience witnesses the murder early in the film, and the detective plays cat and mouse and tricks the guilty party into revealing his guilt. Plus, it stars future Columbo alumnus, Ray Milland.