Tater Patch
The tune is Tater Patch. Repeated AABB (after a while).
Key of A. Two parts. The longer high part is played twice. The shorter low part can be played twice or thrice. So, the song structure is AABB or AABBB.
Tommy Jarrell in Wayne Erbson's field recording [DLA] (AABB).
Hear this on the Highwoods String Band's best album [clip at Amazon] (AABBB).
Della Mae live bluegrass [tube] (three second parts AND jump the last beat!)
Vi Wickam tune a day [tube] (AABBB). Skip to 0:35 for music.
Fiddle instruction [tube], one of several slow-downs by the same instructor (AABBB).