After serious thinking newshound wrote :
Interesting. I guess it is just meant to be on-off.

There has to be some initial slippage and wear, when it engages with
the engine running at full speed, so not quite on off.

Provided the load is high
enough it probably never really slips. I wonder if it is hardened steel, or
just mild steel? Ordinary grey cast iron would probably work OK as the
graphite might help to prevent the metal picking up or scoring.

The metal plates seem to be just pressed mild steel and both are quite
scored. I have had a long term issue with my grass being quite tough
and moist, jamming up the cutter deck - probably causing a lot of
clutch slippage.

I earlier ran a full test of my 'gasket material' modified clutch and
it seems to be holding up.