Circa 1913 Knutsen Harp Mandolin
Chris J. Knutsen was a luthier who worked in the Pacific Northwest and Los Angeles in the early part of the 20th century. He is not well known in the wider guitar community even though he basically invented two famous styles of guitar, which, due to a run of complicated circumstances, are not known under his own name. The first is the Dyer Harp Guitar, which is named for the man who distributed them. The other is the Weissenborn Hawaiian guitar. (You can read about the Knutsen-Dyer situation here and the Knutsen-Weissenborn story here. For more info on Chris Knutsen, you can go here.)
Along with guitars, Knutsen also built ukuleles and mandolins, including the lovely ebony example here. Usually, Knutsen’s mandolins had the extended body but only five of them are know to have the extra sub-bass strings. I’m not sure how useful the extra strings are on a mandolin. Harp guitarists tend to play fingerstyle and use their thumbs to hit the bass strings. But mandolinists use a flatpick and to my mind the typical tremolo technique would make difficult to hit the bass strings. But if you want to prove me wrong you can send $5500 to Harp Guitar Music and work out the technique. For more info on harp mandolins, harp ukuleles and harp guitars, head on over to Harpguitars.net. Make sure you set aside a large block of time. There is enough amazing information to keep you intrigued for hours.