The Octave mandolin or Bouzouki is tuned one octave below a mandolin. It can have several scale lengths, different tunings and configurations of string.
This particular one has a 23 inch scale and four courses of strings. A tenor banjo or even a 5 string banjo set up would also be possible on this instrument.
Although its ancestry is in the Eastern Mediterranean today it is also used as an accompanying instrument for Celtic music.
This acoustic resonator version of the instrument gives the punch and volume boost that can usually only be obtained with a pickup.