Cale on his Harmony, from the Gibson website:
It's pretty much inoperable now. The airlines messed it up pretty badly. That was one of the reasons I kept modifying it, or trying to fix it. In those days I didn't know as much about modifying guitars. I took the back off it, because an airline had crushed it. Also, it was an acoustic guitar, and we were starting to play bigger gigs, and I needed to play louder. This was before piezo pickups. When I put a magnetic pickup on it, I would get up to a certain volume level, and then it would feedback. In those days there wasn't a good way to prevent that. One reason I took the back off the guitar, and put a rod through it, was so it wouldn't generate feedback any more. (Full interview here)
Other implied guitars: any number of Eddie Van Halen's home made guitars , The Les Paul 'Log', and Brian May's 'Red Special' or 'Fireplace.'