Carolina Chocolate Drops: Old Rattler.
Key of C. Chord progression is 4 C strums then 4 G strums, repeat. The chord switch comes just before the end of the phrase:
Grandpa Jones [utoob]
Carolina Chocolate Drops [utoob], original lineup, with fiddle & banjo players having switched instruments.
This tune is on the Gravy Soppers CD, Live on WPAQ.