Green’s Dictionary of Slang

a harmonica, a Jew’s harp.

1884		(con. c.1840) ‘Mark Twain’ Huckleberry Finn 363: I ain’ got nuffn but a coase comb en a piece o’ paper, en a juice-harp.
1906		Hull Dly Mail 8 May 1/7: ‘How would you imitate the “music” of a cider press?’ ‘With a juice harp’ answered the poet.
1967		(con. 1920s) J. Thompson South of Heaven (1994) 74: A guy was twanging on a juice-harp.