Hello John,  I still have and play my Triumphauto-London that
I restored about 20 years ago.  I have about 50 music rolls that use
the Themodist feature, which works well.

I made three simple jigs which firmly hold the two boards at the proper
spacing from one end while you are covering the other three.  I loaned
the set to a friend who, by the greatest coincidence, also had a piano
with this same action.

The parallel boards provide twice the force for the same area, if you
think about it.  This reduces the front-to-back dimension of the player
action.  For what it's worth, here are some "tricks" that I used.

For the jigs, a long block of a thickness matching the desired
separation of the two boards will give you a place to mount two clamp
fingers to grip the boards.  A through-hole, bolt and wing-nut will
tighten both fingers at once.  This jig lets you cover the two sides
and one end.

To strip these small boards without losing any wood and changing the
dimensions I did the following:

First I prepared two pine boards (sized to fit inside the kitchen
oven) with a set of nails that I had driven right through to provide
a perch for each pneumatic board, using its existing screw hole.  This
was needed to facilitate the forthcoming drying process.

I stripped all covering from the pneumatic boards by dropping them
into a large shallow pan of roiling boiling water.  Both the coverings
and the glue are removed in a matter of seconds!  To keep the pneumatic
boards from taking up a significant amount of water, I immediately
placed them in an oven set to 225 degrees F.  As soon as dry, they were
removed.  They did not warp, since everything was done quickly.

I used about 1/4-inch overlap for the covering.  I was concerned about
the higher level of mechanical duty on the overlap, as compared with
the hinge-end of a hinged unit, so I use a flexible white glue (PPCo
320), but only on the overlap.  None of these units have ever failed.

If you are not installing a vacuum supply, it doesn't matter, but with
the Themodist feature I found it necessary to have a different vacuum
level when that feature is selected.  (I automated my vacuum system to
account for this third extra level, beyond play and re-roll.)

Best regards,
Bill McKeown
Euless, Texas