My guess is that Robbie's answer to Jeff Bridges' question, "Why are
tracker bars made of brass and not of aluminum," is probably correct.
Tracker bar construction involves much soldering, and brass solders
easily. But also keep in mind that, as construction metals go, aluminum
is a fairly new one, and early extraction methods made it very expensive
to use. Alcoa made its fortune around the turn of the 20th century by
mastering the technology.
The John Adams Building of the Library of Congress, built in 1939, used
several new and modern materials in its construction. The metal used
to decorate the walls of the building was aluminum, laid against some
vinyl-looking green wall covering. Nobody gives that aluminum a second
glance today, but in 1939 it was a novelty.
Clear acrylic plastic (e.g., Plexiglas, Lucite) has been used in at
least one case to make a tracker bar. Its advantages are, besides
cost, ease of machining and the ability to see where the holes you are
drilling are going. That plastic tracker bar is on a DeKleist band
organ playing style 165 rolls, owned by Art Curtze of State College,
Irondequoit, New York