Both Rev. Gary Davis and Merle Travis would make a C7 position chord(anywhere on the neck from the first fret up) with the thumb covering the 5th AND the 6th string. This freed up the pinky to cover the 1st string on the same fret as the thumb. It took me a long bit of work to be able to do it but enables me to play a closed position C7 anywhere on the neck.

Mark Ross