Robbie, you got it: the tracker bar, wood or metal, indeed is the stick
where, one way or another, the keys are connected with the valves.  This
can be achieved by using air (like in most organs), but also using levers
which raise when they do meet a hole in the source.

A remark must be made: I did use the book "The Marvelous World..."
together with other sources to write you my text.  So it is not a "text
from the book". However, the classification-scheme (that with the
"-phones") is copied from the  book.

Jan

 [ Thanks, Craig and Jan, it's beginning to make sense now.  -- Robbie