I have always thought that Gulbransen built their own upright pianos.
They did not make the plate as few manufacturers of pianos actually
cast their own plates.  In the heyday of the piano manufacturing
industry there were many vendors that supplied the piano manufacturers
with parts.

The piano actions were made by Wessel-Nickel-Gross or Pratt-Read and
others.  The plates were by Wickham, Kelly, Superior, and others.  The
nine-foot Knabe pianos, at one time, were made by A.B. Chase for a year
when Knabe sales were so good that they could not meet the demand for
their product.

This practice of buying from different vendors is still practiced today
by many piano builders in the USA.  I doubt that Gulbransen made their
own nine-foot pianos as they are very rare today.  It would be
interesting to find out who actually made their pianos.  Seeburg, for
example didn't start making their own pianos until the early 1920's.

The above is just an opinion based on 40 years in the music industry.

Don Teach