Regarding Steinway Duo-Art installations -- I've seen several 1928
Duo-Arts with the fan-accordion expression system which Bowers
Encyclopedia of Mechanical Musical instruments says were introduced
in the mid 1930s.

The fan system has at least three major variants, and it was
installed right up to the end, at the same time as the standard Duo-Art
expression system.  It does _not_ seem to me to have been developed
as a replacement or an improvement for the standard Duo-Art expression
system, as the Model B Ampico seems to have been for the earlier A
Ampico.  As the first installations of the fan-system Duo-Art appeared
in 1928 pianos, development had to have been in 1926-27, thus it is not
the brain-child of 1930s Ampico technicians.

The exhaust holes in the zero pneumatics in many of the fan systems are
square: for the Accomp. measuring 1/2 x /12, and the Theme measuring
1/2 x 3/4, sometimes 1/2 x 5/8.  They are purposely different sizes,
something which seems to mimic the partly covered exhaust hole in the
Accompaniment side of standard expression systems, which was obviously
not a design error as sometimes has been postulated.

One variety of the fan system -- the large square one which appears to
have been designed to fit in exactly the same space as the standard
square expression box -- has both Accomp. and Theme zero exhaust holes
the same size as the standard Duo-Art expression boxes, not the tiny
holes in many of the other fan systems.

Interesting to me is that most fan systems, though not all, only close
the spill as the Theme registers, thus giving the higher Accompaniment
powers floating values.

There are some variations in the standard Duo-Art expression boxes
(like no crash) which, when coupled with being often in smaller size
pianos with smaller striker pneumatics, may have been a purposeful
attempt to fit the piano to the size room it was destined to play in.

On the other end of the volume spectrum are the 9' 6" Steinways which
were set up to play concert halls.  They had two pumps, etc.  I could
go on for pages, but all of this has been covered in detail in
published articles in the AMICA Bulletins.

I am currently running three of these fan systems.  You can find more
lengthy descriptions and many pictures in AMICA Bulletin articles I
have written.

Bruce Grimes