It is incorrect that playback speed of "Robert Johoson's CD" is. This song was recorded at speeds of 74~5 rpm, not 78rpm. So this song is Key of A, not Bb (, and he had played this song by National Reso-Phonic guitar (tri-cone?), not Gibson.
Robert Johnson's recordings were apparently sped up past their original speed, raising the pitch aswell. Many listeners may have been listening to him at the incorrect pitch and speed. Listen for yourself. I personally think that this song sounds more natural at this pitch, and much bluesier giving a lot of credit to the story.