The accordion was used a lot in the background and theme music for "The Beverly Hillbillies", which featured music, and occasional appearances by "Flatt and Scruggs", and was how a most Americans became familiar with Bluegrass music in general.

The accordionist was Leo Mannes, who used the stage name "Zeke Manners"--he had been the founder of the original "Beverly Hillbillies", which, if you don't already know, had been a "Hollywood Cowboy" band, playing on radio, in films, and recordings, in the early 1930's.

He played and performed with Roy Rogers, Elton Britt, and the like for many years, and worked as a musical consultant for films and TV--I am not sure how relevant his work was to "Real Bluegrass", but he did create the sound that passed for hillbilly/western music in movies and TV--