The Dixie Hummingbirds. Looks about late 1950's.
The Fairfield Four has been a favorite of mine for twenty years. The Nashville Bluegrass Band does the two-minute intro, but the arrangement is all FF4. (Yeah, they're the ones who did Po Lazarus in "Brother Where Art Thou," one of the few movies I've seen.)
And The Blind Boys of Alabama - a little different flavor.