Murray,

I was suspecting that Sonny Terry may not have carried a complete chromatic set of harps. If you are tuned to C instead of E then an E chord formation yields C. A G chord yields Eb (cross harp G#) A D chord yields Bb (cross harp F). An A chord formation yields an F (cross harp C). If you tend to travel with just a few basic harps, you might miss many of these. Of course, this is pure speculation on my part.

Roger in Baltimore