Mountain Dew
I just watched Three Brides for Hoss (1966), and two brothers of one of the erstwhile brides were reminiscing about the good ol' days in 'Tucky, and in particular what they referred to as "Mountain Dew." They repeated the name three times, which made it occur to me that this was actually product placement in the script.
The very first color Mountain Dew commercial was also from 1966:
https://www.youtube.com/watch?v=A_eSUBotF64
I'd be willing to bet Mountain Dew was a corporate sponsor of Bonanza (1959), and that the commercial made its television debut accompanying that episode.
§ "Precisely the point of a lonnnng dinin' table."