Niels Berkers wrote of his player piano in which some of the tracker
bar ports were not utilized [050228 MMDigest].  I had a Hardman Duo
player a few years ago, one of the early ones that didn't have the
plastic valves, and it also had an un-utilized tracker bar port.
In this case, there was a sustain port, but no tubing to it, and no
sustain pneumatic.

I was told that, as likely as not, when Aeolian-American started
building these after WW2, they probably had a stock of old tracker
bars left over, and utilized them, but omitted the sustain pneumatic
for whatever reason (I cannot imagine that adding and tubing in a
sustain pneumatic in a factory setting would be all that great an
expense).

I'm not sure if this hypothesis about leftover tracker bars is true,
but I am aware that the early Hardman Duo players all had tracker bar
sustain ports that went un-utilized.

Bryan Cather