Further to Beatrice Robinson's posting about a small electronic
locomotive, a similar device can be seen in The Organette Book, Fig.
575.  This is called 'Go-Go Tunes' and plays an eight-note diatonic
scale with the aid of press-in pegs placed into the holes in the track.
Any simple melody can be programmed.  The loco has eight spring-loaded
keys on its underside and is powered by two AA cells.

The 'Blow a Tune', described by me in MMD 040703, is shown in figures
569 to 572.

In terms of the smallest number of notes in a mechanical music
instrument, the 'See a Tune' may be the winner.  This has only four
notes, C,D,E and F, and operates with metal balls being dropped onto
metal bars.  It is shown in Figures 576 and 577.

For this discussion thread I am excluding the large range of hand
cranked chord-playing reed instruments, such as the 'Chein Church' or
the rarer 'Notre Dame'.  These are sometimes called 'Amen Boxes' due
to the chord progression.  These, although being a fun addition to
a collection, cannot strictly be classified as mechanical music as they
do not play a tune, but rather a set of linked chords, usually three,
each consisting of four notes.

Best wishes from GB
Nicholas Simons