Just as an off-the-cuff reaction, the following thoughts spring to mind:

- very early country music (the sound track to "O Brother, Where Art Thou", although some have described it as 'proto-bluegrass')

- songs like "You Are My Sunshine", "Cripple Creek", "Old Joe Clark", "Keep on the Sunny Side", "Bury Me Beneath the Willow"

- Carter Family stuff

- lotta Appalachian mountain songs

- most songs that feature mandolin and frailing (clawhammer) style banjo (as opposed to bluegrass style)

Not claiming or trying to be definitive, but any of that help?