I have some Harden-Temptrol shower valves for which I am
having a hard time finding replacement parts.

From Symmons installation and service documentation for
Symmons Temptrol shower valves, and from photographs,
it appears to be an almost identical design.
I also noticed that Symmons has owned the "Temptrol"
trademark since 1968 (long before my Harden valves were installed).

Did Symmons make the valves for Harden Brass?
or was there some other common corporate history?

But the most important questions is:
Will Symmons Temptrol replacement parts work in these
Harden-Temptrol valves?