My brother got one years ago in Austria. Six strings tuned as a guitar, and a sort of s-curve in the head so another, longer, six-string set miss the neck. The body is lute/potato-bug shaped, and there's a nice 19th-century looking head carved at the top of the neck. No maker's mark. Looks strange & wonderful and I think that's a theorbo.
We supposed you use the strings with frets like any guitar, and the mid-air strings as bass notes. A sort of harp-guitar, apparently. (We couldn't find "Theorbo for Idiots.")
I don't know how much progress he's making with it. He's in Ecuador now. That's my fiddle-footed brother.