The Tracer of Lost Persons, a 1906 episodic novel by then-best-selling novelist Robert W. Chambers, provided the source material for one of old-time radio's longest-running detective shows. As Chambers put it, Westel Keen "was thirty-three, agreeable to look at, equipped with as much culture and intelligence as is tolerated east of Fifth Avenue and west of Madison. He had a couple of elaborate rooms at the Lenox Club, a larger income than seemed to be good for him, and no profession."
Frank and Ann Hummert developed the radio program, taking Chambers' character and turning him into an older sleuth. In the novel, Mr. Keen was a matchmaker for hire; now he was an actual tracer of lost persons and, later, solver of murders. Basically, the entire show, save for the title, was the creation of the Hummerts. The show ran from 1937 to 1955, beginning with a half-hour weekly format on then NBC Blue network, then moving to CBS in 1947; for the last few years of its run, Mister Keen, Tracer of Lost Persons switched to a 15-minute weekday program. (Again, information varies. The first episodes may have been fifteen-minute ones and the last few months of the program's run it may have reverted back to half-hour episodes. Most likely everything has been explained in Jim Cox's 2004 book Mister Keen, Tracer of Lost Persons: A Complete History and Episode Log of Radio's Most Durable Detective, which I have not read but you are more than welcome to.) Over the eighteen years of its existence, 1690 episodes were broadcast nationwide; of those, only 59 episodes are known to survive.
Bennett Kilpack starred as the aloof Mr. Keen (who, as far as I know, had no first name in the series) until October 1950, when he was replaced by Phil Clarke and later by Arthur Hughes (or perhaps it was the other way around). Jim Kelly played the not-too-bright, stereotypical (for the day) Irish assistant Mike Clancy.
Enjoy this episode, which appears to be the last surviving one featuring Bennett Kilpack.