This from "Harp of Glory" an article which appeared in Acoustic Guitar #22 Jan./ Feb. 1994 by Jody Stecher, a very fine musician in his own right, who met Spence in 1965:

"He tuned his guitar at standard pitch or lower, often a whole tone down. When playing in D, he lowered the sixth a whole step, to an octave below the fourth string. Spence employed a rather wide gap between the treble and bass voices of his instrument. I can approximate this sound by increasing the gap between my fouth and third strings to an uncomfortably wide fourth interval and then tuning the remaining two high strings to play perfectly in tune with the now slightly sharp G. It sounds a lot like Spence and it's very hard to get back into normal tuning- the whole guitar goes bonkers . This is a dangerous tuning: don't try it at home."

Frankie