Richard Bridge suggests that Leadbelly tuned down two tones(bottom string to C). I have also seen other suggestions, that he commonly tuned down to D. Maybe he varied. What I am certain of is that in the Midnight Special clip I linked to earlier he is playing in A-shapes, and it is coming out in the key of A.You can be sure of this from the distictive sound of the open E A and D strings. ie he is either playing on a conventionally tuned guitar in A, or in a down tuned guitar capoed up as many frets as he tuned down in semitones. The latter is quite likely I would think.