Ray Finch asked about octave coupling.  It is where the notes of one
octave being played are connected to those in an upper or lower octave
so that both play together.

In pump reed organs there are commonly octave couplers in the treble
which link the central octaves of the keyboard one octave up and
likewise in the bass that link the notes downwards.  This means a
better musical effect may be obtained with less effort by the
performer.

In certain coin pianos there is a limited musical scale available on
the roll.  The pneumatic stack is larger than the roll's musical
compass and the top or bottom octaves may be coupled to higher and
lower octaves to provide a fuller musical effect.  In practice all it
requires is an few extra bits of tubing inserted.  One bass note hole
on the roll will play two bass notes one octave apart on the
instrument.

There is also a piano with octave couplers (operated by an extra pedal)
called a Moor Klaviatur.  The mechanism is very complex and they're
very rare.  I had the fortune to play on one a few years ago.  Does
anyone know anything about them?

Adam Ramet