Some years back a random job-lot I received contained an "Interchangeable
Tracker Board" for Cecilian No. 4979, in its original, carefully-labelled
box supplied by the Farrand Organ Company.  The tracker bar is of the
irregular-spacing design described in earlier posts.  It's beautifully
made from wood, with a birds-eye maple veneer on its upper surface and
the serial number stamped underneath.  It carries a patent date of 13
November 1900.

However, exactly what it was to be interchanged with?  Another identical
tracker bar, or possibly a standard 65-note one?  I've never encountered
an explanation, nor examined an instrument to check it out further.

There is at least one of these instruments restored and accessible,
in the playroom of the Frank Lloyd Wright house in Chicago.  It was
installed there in 1982 to replicate the original that Wright's son
recalled.  It was restored by Mel Septon, and sits besides the Steinway
grand that's rather quirkily suspended over the staircase.  You can see it
at, for instance, http://www.flickr.com/photos/ghosstrider/7970785498/  or
two thirds the way down http://www.rachelleb.com/tag/frank-lloyd-wright/
I can even see that it has a tracker bar just like the one sitting here!

Julian Dyer