I have only seen four or five of the Western Electric Derby model
pianos.  The drive sprockets from the pump shaft to the roll frame are
usually a leather sprocket.  In some cases I am sure they were replaced
as they had been replaced on one of them that I restored.  We just made
them new as we could not locate a source for them.

I have one early model Derby in the warehouse that may be different,
but to be honest I have never really looked at it very close.  I know
the race horse part is a little different from the others I have seen.

Don Teach