Some late model Duo-Arts have only one location for the serial number
and that is on the plate.  Once painted over, that number is not found
anywhere else on the piano.

After the Aeolian sales slowed,  a policy of not assigning serial
numbers at the Steinway factory was adopted for pianos destined for
Aeolian and Du0-Art production.  As Aeolian took these pianos out of
their storage warehouse, numbers were assigned by Steinway.  In some
cases, the piano might carry a 1932 serial number even though the
piano left Steinway in 1929.

All Steinways are marked with case identification production numbers.
These numbers appear on most case parts and these numbers may be
crossed referenced in factory records to reveal the serial number that
was assigned to that piano.

Bob Taylor