From Sandy Paton's notes to the song on the Folk-Legacy LP "Frank Proffitt, Reese, N.C." (1962):

"Frank's grandfather, a great admirer of Abe Lincoln, chose to go the hard way--he went 'across the mountain to join the boys in
blue'. (Once, separated from his outfit, he wandered into a camp of Confederate soldiers and found himself face to face with his
own brother, who had made the other decision. By feigning insanity, he managed to escape and rejoin his own unit, but one can
well imagine the anxious moments experienced by the two brothers
in the interim.} Frank learned this song, which describes the
farewell of a 'Southern Yankee' to his sweetheart, from his father.
Frank is not sure, but it seems likely that his father had it, in
turn from his father, I have not found it reported elsewhere."

Hopes this helps: it's from a year earlier than the Folk Festival.