I dug up some more information concerning the Centripetal Tension
Resonator in Mason & Hamlin grands.  This comes from Alfred Dolge's
book, "Pianos and their Makers", published in 1911 by Covina Publishing
Co., Covina, California, pages 109-111.

The "Tension Resonator for Pianos" was invented by Richard W. Gertz
and was patented October 2, 1900.  Illustrations include the underside
of a large grand piano with a resonator having twelve tension rods and
views of how the device is constructed and manner in which tension rods
are attached to the soundboard rim.  It is stated, "This invention has
been applied to all the grand pianos made by Mason & Hamlin since the
granting of the patent."

Jeffrey R. Wood