In the 030110 MMDigest, Jim Welty spoke of a Melville Clark player
piano capable of playing any of five different roll formats.  Okay,
I've never seen one of these, so I'll ask:

1. What were the five different formats?

2. How do these work?

I assume it's a tracker bar with five different sets of horizontal holes?
But how is only one set selected?  (How is it implemented: internally?)

3. Is it practical to retrofit an existing player piano to play these
five formats, assuming that the parts are available?

Cheers!
Jim Cook
Bolton, MA