That's it, here's a close up:
Attachment:
IMG_3729 reduced.jpg [ 149.94 KiB | Viewed 2259 times ]
This one's pretty worn and hardened.
The surface would be dimpled to help with grip. When I first saw this it reminded be of the rubber surface on the table-tennis bats we had as kids. Those dimples were a lot bigger from memory but it did get me thinking of a starting point for a substitute since I assume the part is no longer made and there's a finite supply left. At a guess I suppose matching the thickness might be the most important part to keep the mechanism working properly?
When I get my hands on a genuine part I'll photograph it, measure the thickness with some callipers and keep that info safe for a day when maybe I can't get the part any more.