Our Rollography, Volume 2, has a good section on Recordo systems,
including the Cable Euphona variant.  Just about the only common thing
between the many different realizations of the Recordo system was the
tracker bar, which is readily identified by the 5 elongated ports at
each end of the normal 88 note-holes.  Briefly, the functions are as
follows:

Counting the 98 ports from left to right:

 1  Bass hammer rail
 2  Mandolin rail  (Motor shutoff in some versions)
 3  Damper
 4  Repeat
 5  Rewind

94  Expression level 4
95  Expression level 3
96  Expression level 2
97  Expression level 1
98  Treble hammer rail

In the Cable Euphona (and most Recordos) the 5 expression levels are
over-riding, in which a higher level port determines the stack vacuum.
Typical vacuum levels in inches of water are:

 PP  Level 0     9 inches water
  P  Level 1    12
 MF  Level 2    16
  F  Level 3    21
 FF  Level 4    35

These levels are good starting points, but each piano should be
adjusted so it sounds right.  The steps should sound equal, with
the hammer rail dropping the perceived sound by about half.

There are two additive types of systems, both 16 step, that add the
steps in a binary mode, but they are rarely seen.  Most Recordo pianos
are the type described above.

Bob Billings