There is another place one might find the serial number of a Duo-Art
Steinway, and that is under the keybed, often fairly close to their
case code stamp, usually the treble end.
The reason for stamping a Steinway with a serial number somewhere on
the bottom, was because they were all hand-made, and a bit different,
from one to another. Once Aeolian mounted the lyre brace, the chances
of that fitting another Steinway was slim and none, because the brace
often was mounted on the stack in early forward-facing stacks, and on
a special block placed in such a way that it would clear the stack in
late models. Even at that, I have seen a few bent braces designed to
clear the stack. I have found in parts collections, braces for
Steinways of about the same model and year that didn't quite fit
another Steinway which had lost its brace, So I assume they were
individually fit. Usually, the angles of the end tabs of the braces
are different. (I don't know this to be the case, however, but it's
a reasonable assumption from my experiences).
Since serial numbers determine Steinway's liability to a customer,
every piano sold had to have one, even during the depression when
pianos weren't selling. That's why they had to give Aeolian their own
current serial number. But I have never run into even the latest models
of depression Steinway M Duo-Arts that didn't have an embossed serial
number duplicated somewhere under it, but all pianos are a little
different in one way or the other, so I wouldn't say for sure that a
few didn't come out with no embossed serial number. Still, if it had
the serial number stamped on the plate and that number had been painted
out, then that number is still recoverable.
I have checked with the Steinway factory in regard to case numbers, and
they have always told me they have no cross-reference to serial numbers
from case numbers. As I understand, it was the year the piano was sold
during the depression that determined its serial number (which have
never been numbered according to case codes, styles, or anything else,
anyway. It's just a contiguous number system of record).
I have even tried to find original ownership records of special order
artcase Steinway Duo-Arts which obviously had to be taken care of by
Steinway, directly. But if they were a Duo-Art, it seems Steinway will
have no record of it as they do with their regular pianos. This is
because Aeolian was the end point distributor.