on the subject of modified guitars:

Big Joe Williams – Nine String Guitar Blues
Delmark DD 627
“Any description of his guitar is inadequate. It must be thoroughly examined to be believed. It is a big flat-top Sovereign acoustic, amplified as an afterthought. Masking tape holds the body together and keeps the pick-up from falling out. An extra set of pegs is attached to the top of the head, and the holes are drilled in the bridge to accommodate the three extra strings. These double the first, second and fourth strings. The guitar is tuned in G (open tuning): d-g-d-d-g-b-b-d-d. Joe plays with the first, sometimes the second fret capoed."

Thomas.