Thank you to all. I had no idea. Now I know where and how to fit it.
Peter.👍

Hi Peter.
It's a 'Kent Clear-view Screen' which is fitted to the outside of the windscreen and has a spinning outer disk that throws water off because of centrifugal force. This is much more effective that a conventional wiper blade where there is a large volume of water to clear as you would find in heavy sea conditions and heavy rain.
The disk is driven by a motor and belt, or by direct drive in modern units, on the inside if the windscreen.
There's a bit on WikiPedia here: https://en.wikipedia.org/wiki/Clear_view_screen
I hope that's helpful.
Robbob.

Generally known as a Kent screen or clear view screen, it goes on the outside. Rotating within the rim is a circular piece of glass that is spun by an electric motor, throwing off any rain/spray. The arm supports the spindle. (not sure how these pictures appeared as I didn't add them, but they are correct!)

Hi there, its called The Kent Clearview screen the glass on the outside rotates at quite a speed and disperses the water

Hi Peter,
I am not certain but i think it is a rotating window that can be spun by hand to give a clear view if it gets sea spray over the windows.

Hopefully i am correct.

Martin555.