Hi Robbie,  You added a couple of words to paragraph 1, sentence 3,
below, which unfortunately changed the whole meaning, making it
confusing.  Editing the editing, I would add another 2 words,
making it read:

  There was normally a cutout pouch block that was
  supplied with suction only during play, (etc.)

As originally submitted:

  There was normally a cutout pouch block supplied only
  during play, (etc.)

The pouch block doesn't supply suction at all, but is supplied
with suction from a connected source, such as a pump or reservoir.
Here's how it works:

On one side of the pouch is a shallow well with a hole at its center
connected to the theme primary valve block, which receives _no_ suction
during reroll.  On the other side is a shallow but mound-like well with
the high point at its center, where there is a hole tubed to one side
of the tracking pneumatic.  Off to the side of the mound is another hole
tubed to one of the tracker ear valves.  As long as the double tracking
pneumatic gets its suction from a constant source, during reroll the
pouch is going to act like a blank roll covering a tracker bar and not
admit any air to the tracking pneumatic.

However, when unregulated suction is applied to the _other_ side of
the pouch, as it is during "play", (delivered through the theme primary
valve block), then air admitted though the corresponding tracker ear
valve is going to push the pouch aside and continue along to the
tracking pneumatic.  To control movement in both directions and permit
uninterrupted centering during reroll, the block actually contains two
of these pouches.

Please note that earlier instruments may have an additional and larger
block containing valves for automatic tracking -- in which case the
cutout pouch block is tubed to the tracking valve block rather than
directly to the tracking pneumatic.

Regards,
Jeff Wood

 [ Thanks for the added explanation, Jeff.  I understand it now,
 [ I think.  -- Robbie