"This is the Mutual Broadcasting System."
I remember it well. I've got some recordings of The Mutual Radio Theatre. They were quite well done. (The CBS Radio Mystery Theatre was my favorite, however).
Mutual Radio Theatre originally aired over CBS as Sears Radio Theatre. I really preferred the Sears/Mutual show to the CBS Mystery as it had more variety, with comedies, Westerns, romances and sci-fi in addition to the who-dun-its. (That, and CBS's producer-director Himan Brown, both in the old days with Inner Sanctum and on the CBS show tended, I thought, to bring out the "ham" in his actors' performances.)
Mutual also had earlier a drama series called either Hollywood Radio Theatre or Zero Hour, (I've seen it listed under both titles,) which they got second-hand after a syndicated run; they were narrated by Rod Serling and were almost all mysteries. Very good also.