I've seen one MANY many yeas ago as a curiosity in a music shop. The guy there said (as was explained above) that it's intended use was as type of 'slide' guitar - however, he said they were most often purchased to teach young players what each note was when the a string was pressed on any given fret - thus new learners could learn to play "note-by-note".

Dunno how accurate that was - I was about 15 at the time (so around '73)and the guitar I saw LOOKED old then.

Muttley