Mr. de Boer's remarks about converting a 65 note (6 holes per inch)
instrument to 88 note (9 holes per inch) raises again a question in
my mind that has bugged me for years.

I have never seen any piano that was made to play both formats, so my
question is: is it necessary for such an instrument to have two tracker
bars?

I have always assumed that the people who designed the modern 88 note
format chose 9 holes to the inch rather than the more rational 10,
because the big punched holes in 65 note music will line up with 2 out
of every 3 holes in an 88 note tracker bar.

If the tubing is switched internally so that the 88 note tracker ports
are connected thus ( o=connected, x=unconnected ):

o x o o x o o x o
1 - 2 3 - 4 5 - 6, etc. for 65 note rolls, and...

o o o o o o o o o
1 2 3 4 5 6 7 8 9, etc. for 88 note rolls...

One could use one tracker bar for both formats.  I just confirmed that
all the necessary ports in an 88 note bar are almost fully uncovered by
the wide holes in a 65 note roll, by laying such a roll over my Ampico
tracker bar.

One would need some sort of internal switching mechanism; either a pouch
relay array or a sliding switch block, to make the transition, along
with a swing-down or slip-on set of chuck adapters.

Both roll formats use 11-1/4" wide paper, even though that leaves a
wider margin on an 88 note roll than was customary at that time.  Lucky
for later developers of reproducing pianos that this is the case, but
it is another clue that the original designers of the 88 note format
had back-compatability in mind.

Can any MMD member confirm that such a switchable 88/65 tracker bar was
actually used?

Richard Vance