In the photo at https://www.mmdigest.com/Gallery/Pictures/herr01.html 
Randolph Herr describes a mystery pneumatic in his Red Welte Vorsetzer,
and concludes that the only thing it could possibly have been was
a tracker bar pump.  I believe he is correct, after seeing something
similar (but not attached to the piano) on this page (scroll down to
see the pump):

http://www.mtfca.com/discus/messages/118802/119786.html?1263608939 

You can see that the tracker pump pictured on that site looks nearly
identical, in its general appearance, to the tracker pump pictured in
the MMDigest Gallery article.  Therefore I think it is safe to conclude
that Mr. Herr is correct in his deduction of what the pneumatic in his
Vorsetzer is used for.

Luke Myers
ldmyers95@gmail.com.geentroep [delete ".geentroep" to reply]