Harry Bloomfield presented the following explanation :
Lawnflite 703 Autodrive. PTO clutch for deck drive was slipping, so I decided
to have a go at it. I have looked at it before, a mechanically operated
clutch (rather than electrical) and then I found it was simply metal face, to
metal face - no sign of any friction material, but it worked, so maybe that
was how it was designed to work?

Bump, someone must know?

Is a clutch ever designed to operate without any friction material just
steel against steel. There were no obvious signs of any friction
material ever having been there..