As David C. Carter mentioned above, during the Great Folk Scare of the sixties, Spider John didn't play a 12-string, but a converted 7-string. He put a banjo peg in the center of the tuning head and added an octave G string. This is the guitar he played on "Spider Blues", "Blues, Rags & Hollers", "Lots More Blues, Rags & Hollers", "The Return Of Koerner, Ray & Glover" and probably others as a side man.

-Roger McGuinn had Martin build him a 7-string guitar inspired by Spider John's 7-string.

-In the 1930s, George Van Eps had Epiphone build him a 7-string with an added bass string.
-A few decades after that, Lenny Breau had Kirk Sand build him a 7-string guitar with a high A string added to the six regular strings.

So there are at least 3 incarnations of the 7-string guitar, those popularised by George Van Eps, Lenny Breau and Spider John Koerner.