I am trying to find a source for parts for a old Supamix shower unit.

The part I require is a small plastic piece with a brass lefthand threaded
shaft running through it. This is used via the main control knob to control
the water flow.
I believe it may be called the plunger plate.

I have a drawing of the unit and parts I need plus photographs of the parts
required if you can help me.

Thanks

Victor Tait