This is a picture of the guitar Spider John used on the Blues, Rags & Hollers and the Lots More Blues, Rags & Hollers albums in the sixties. These were very popular albums back then and I can't believe that Roger McGuinn (Jim McGuinn in those days) wasn't familiar with them. I think John made his own 7-string by putting a banjo tuning peg on the peg-head of a six-string with a tailpiece.