This guitar was made in 1916 which was the last year the founder of Gibson personally oversaw the production of thee guitars. It is 1 among (approximately) 6 remaining of its kind. On Jeff's model, there are 10 harp strings which are used primarily for a droning effect or harmonics.
The idea behind the harp guitar was to give players more bass strings so that they could utilize them with odd chords. They could play a three or four-string Eb, for example on the regular neck, and then add the bass not from the sub-bass strings.
Gibson offered harp guitars from the company's founding in 1902 all the way to World War II, but it is unlikely that any were made after the 1920s. Gibson was not the only maker of harp guitars; Carl Knutsen designed a flat top harp guitar, and the Larson Bros. perfected his design.
Gibson thought the harp guitar would be to the guitar what the piano was to the harpsichord. But it proved a little too cumbersome.
You can hear the harp guitar being played on Shadows on the Mountainside and The Badger from The Edges of Twilight