Jack -- one of the things that Hammond did was notate a melody in D major, and then mark all the Cs as flatted. I can't give a songtitle, because I'm not in the same house as my copy of Marrowbones.

While that's accurate in one way (he notated that the singer consistently flatted the C), it suggests that he was trying to transcribe a tune in D major -- but that didn't fit cos the Cs were consistently flatted. But, to his trained ear, it was clear that the tune wasn't in E minor (i.e. the minor key with a sharp F but a flat C), but somewhere inbetween D major and E minor.

Of course, this isn't Hammond's problem -- he's up against the limits of the musical notation he was using.