MMDers:  I was recently working on a Grand Roller Organ and noticed
that the "keys" that read the pinned roller, are a little loose.
The tips of the keys will move about 0.010 inches (0.254 mm) in either
direction.

The Grand Roller Organ plays a 13+ inch (33 cm) by 2.5 inch
(6.35 cm) diameter wooden roller with 0.035 inch (0.89 mm) round pins.
The pins are on a spiral path at a pitch of 0.050 inches (1.27 mm)
per revolution and the key tip is 0.040 inches (1.01 mm) wide.

I was initially surprised to find that much play in the keys but it
turns out that it is beneficial for the keys to move from side to side
a bit.  If the pinned roller is off enough for the key tip to be just
hitting the next over (wrong) row of pins, the key tip will be pushed
aside and slide off of the pin so that the wrong note doesn't play.
On the other hand, the side to side play is not enough so that the
tip slides off of the correct row of pins.  What a clever design.

I was wondering if there might be examples of other mechanical music
machines that use a similar approach.

Charlie Moore