Long-time listener here, but I’m the first to admit to some gaps in my knowledge of radio history. Oh, sure, I knew that US radio and TV call letters begin with “W” for stations east of the Mississippi and “K” for stations west of the Mississippi*, and that Canadian stations’ call signs begin with “C.” I recognized many call letters as representing the networks that owned or operated them: KABC, WCBS, KPBS. I knew that the call signs of many public-radio stations include the initials of the colleges and universities that house their studios: KFJC (Foothill Junior College), KCSM (College of San Mateo), KPCC (Pasadena City College), WBUR (Boston University Radio). And I appreciated the Bay Area references in many local stations’ call letters: KABL, KFOG, KOIT (for Coit Tower, one of the city’s quirkier landmarks).
I also knew one call sign whose initials stood for a phrase: Chicago’s WGN, for “World’s Greatest Newspaper,” aka the Chicago Tribune. But it wasn’t until I started watching Ken Burns’s excellent eight-episode Country Music on PBS (that’s Public Broadcasting System, in case you didn’t already know) that I learned how many other early call signals—though randomly assigned—took on extra character as initialisms for slogans and phrases used as commercial gimmicks or mnemonics.
Undated ad for WSM, which began broadcasting from Nashville on October 5, 1925.