A fretless zither that has 4 sets of chord strings (C major, G major, F major and D7) which are strummed with the left hand and 2 octaves of double melody strings, (Middle C - C'') which are struck by metal hammers activated by the right hand. The melody strings are tuned to the C major scale, making the instrument somewhat like a piano without any black keys.
The hammers are mounted on spring steel and produce a mandolin-like sound from repeated bouncing on the strings, hence the name mandolin-guitar-zither sometimes applied to the Marxophone. The sound of the Marxophone is also reminiscent of the various types of hammered dulcimer in music traditions worldwide.
The one album credit for a 'marksaphone' has been assumed to be a variant or misspelling as assumed by Discogs. And one forum suggests that one was played by John Sebastian, which would tend to support this.