I'm not sure I understand your question Jacques. The recording is in A, but I'm not sure exactly how that was achieved. My two cents: I think the fiddle is playing in A, because of the sound of the A notes - sounds like an open string to me. I think the guitar is playing in G with a capo, because of the chord choices and runs, and because I don't hear any low notes. I'd be curious to know what others might think.
Thanks Doug and Bob. With these old recordings you never know just from the pitch. But A is the winner ,if only because it's an unusual key for a Mississippi fiddle blues. I just love the Nation brothers ,have slaved long and hard on Bankhead blues. Pity there's so few sides.