Shirley Thomas was a Hollywood reporter for NBC Radio and also hosted her own weekly half-hour program. As listed in Jimmy Stewart on the Air, Jimmy appeared on the July 26, 1953, broadcast. Here is a listing from the Independent Press-Telegram, Long Beach, CA, on the day of the broadcast.
This show is not available and the subject of his appearance is not known. The film Thunder Bay had just premiered in late May, so it is possible he was interviewed about the film.