"My ole mistus promised me When she died she'd set me free." "Good mornin', John. Howdy." "She lived so long her head got bald...." Rest involves her mistreatment: "My old mistus killed a duck, Didn't give me nuffin' but de bone to suck." Etc.
The initial stanzas, about the promise of freedom at the owner's death, is common and supplies the heart of several songs. But all seem to be distinguished by their choruses. It is possible that this is one of the elements that went into the Lomax conglomeration "Old Marse John" -- but it's such a kitchen sink that proof is impossible. - RBW