[Rubbond]

You've got me there Kevin! If Cuppysil is a trade name, it might suggest it
is based on Copper and a Silicone polymer.
I suspect PBC might stand for Pacific Bearing Company, who make high tech
lubricants for the aerospace industry.
I had to revert to the Internet on this one. All I found was a reference to
the US MG club, but couldn't find the relevant text on the site.
A company with website www.traccoatings.com give the following information.
I guess however they are only distributors:
Quote"PBCGR - PBC GREASE Brake Grease

Developed as the optimal grease for braking systems used by many of the
highest rated automobiles. The material is gold in color and is based on an
organo-metallic formulation which does not contain soap greases or
silicones. Can be used up to 1200* C as a torque equalization compound and
anti-seize for studs and threaded components. Also used for roller bearings
and universal joints." Unquote.

To withstand these types of temperatures it couldn't possibly be based on
any hydrocarbon polymer.
With best regards,
Andrew.