Lkeller said:
The first exception to call letters I can remember in station imaging was K-100 (KIQQ) about 1971, but I think Gabbert beat them to that gimmick by a couple of years.
Gabbert did K-101 (KIOI) in 1968...but it turns out that he might not have been the first, after all. In researching 1069_KIFR's comment about KIOO having already been taken when KIQQ launched in 1972, I found something interesting:
A jazz station in Oklahoma City at 100.5 on the dial had the call letters KIOO from 1960 to at least 1973 (they had them when KIQQ launched). A local Oklahoma City radio site shows the call letters as having been used in the 70s, but a search of the Google Books archive of back issues of Billboard magazine show that, in fact, it was KIOO during those 13 years and the site got the calls out of sequence.
Not known: Whether the station called itself K-100 or just let the audience put together that KIOO was FM 100.
The KIOO calls are now in Porterville, California.