Third attempt to post this message:
Vocalion probably used a spring driven cutter, and they were less reliable than the weight driven ones (according to Ralph Peer). If we knew Johnson tuned his guitar perfectly (re: 'A' 440), correct pitching would be a piece of cake. He probably didn't, but still it was probably not too far off, especially if he played with other musicians playing un-tunable instruments (e.g., harmonicas).
RJ NEVER recorded with a harp player; whether he ever worked with one I can't say! However, as a "harp player," I can tell you that I was driven around the bend by trying to sit in with guitarists who were tuned to their own "internal" standards...which usually had NO relation to things like "A 440!"