I've spent over three hours looking for the original folk song on which the mento (etc) version is based. I found many references to the song the OP mentioned, but no song. Mattie Rag/Matty Rag (the mento version) is easy enough to locate, and earliest references seems to go back to 1950. But what song is it based on? Various performers who have done it all say the same thing: "It's an old folk song." My question is this then: Where can it be found??