It sounds like it is their version of a blade brake clutch. It may provide some protection against bending the crank, but the Toro BBC systems could still allow a crank to bend. In those, a belt basically was tightened and made the two half pulleys act as one. So absolute protection, likely not. How much protection would be questionable.

Thanks for replying.
No this is actually a disk shaped like a dish antenna, but smaller.
The blade when tightened bends to the shape of the disk.
The bending actually secures the blade to the disk.
This holds it tight. If it hits a object it slips and protects the crankshaft.
I cannot get a puller large enough to remove.
I think I will have to make something.
I do not want to say anything negative about the UK lawn mowers.
But They were the ones who used positive ground ion their cars and motorcycles. the 60 and 70's.
I was hoping that someone had some experience with British Lawn Mowers.
I hate modifying it but 2.5 inches is way too low for my Black Beauty grass. It likes 3 or 3.5 inches.

They are interesting. Only had the pleasure of working on them once.

Anyway, I got the friction plate off.
I cut the main engine shaft 1/2 inch and I drilled and tapped the hole to make it deeper.
I re installed everything and it works excellent.
Now I can stripe at 3 in and the grass is happy.

This I would love to see, can you load a picture up please of the lawn cut.

Thanks Simon

This is a fancy diamond strype at 3 inches.

Very nice,
Thanks for loading the picture