In the 121007 MMD Bob Taylor asks about some peculiar Duo-Art coding.
While I don't know for sure, I suspect the likely use of the normal
"motor cut-off" hole perforation in Concerto rolls would be to tee it
into the crash valve pouch that opens the bypass channel from the pump
to the theme expression.  The 1927 service manual (p. 44) indicates the
crash valve is designed so that "very quick loud accents can be
obtained" when the accordion goes to step 15 and opens the crash pallet
valve.  But most Duo-Art techs know that it is a little gimmicky, since
in the normal configuration, the theme accordion still must totally
collapse in order to trigger the crash valve.  So it should be no
faster at accenting notes than using the knife valve alone.  And the
real effect is usually that step 15 may be a little bit louder than
step 14 because the open crash valve usually has less pressure drop
than the fully open knife valve.

However, if Concerto rolls were coded for the crash valve, it would
give a very fast pneumatic theme fortissimo without the time lag of the
accordion operations.  Your post makes me wonder if Aeolian may have
once considered coding in the crash valve for regular rolls, but then
never did because the tracker bar standard was already set, and because
many uprights lacked the crash valve altogether.  Anyway, thanks for
asking an interesting question that I've wondered about as well.

Ralph Nielsen
www.historicpianos.com