In 1963, FM broadcasters using stereo encoders were still quite few. This would quickly change as large, cabinet home sound systems with AM/FM stereo receivers began to be sold. A few FM broadcasters new to stereo transmission, would air select programs in stereo while reverting to mono for others. A few holdouts, such as 93.3 KBLE-FM & non-commercial 107.7 KRAB would remain mono until the 1980’s.
I think KRAB went stereo in the mid 70,s. I understand that KBLE-FM was the last commercial FM in mono. Now we have KIRO-FM.