No idea where Monroe got this version, but did ya check out D K Wilgus' article on the ballad and its history in the 1956 Kentucky Folklore Record? (There's another article by Janson in the same journal, 1958, on the topic.) Wilgus was one of the first pro folklorists to take hillbilly music seriously, and it's quite possible he asked Monroe about it, directly or indirectly.