>    The interesting thing about these registering pianos is that they
> were electric, their playback was apparently immediate or within the
> hour, and their mechanism was conceived somewhere around the turn of
> the century...

This may be apocryphal but...

I think I heard that the Welte recording mechanism attached carbon rods
to the keys.  The end of the rods were dipped in mercury, so moving the
key changed the resistance.  As the key was pressed down, an inked,
wedge shaped, rubber roller contacted the master roll.  The further the
key was pressed, the harder the roller pressed, and the wider the
line.

I don't know how playback was accomplished.