Another Mystery Object For You To Identify

As you both know, the family spent a pleasant week in South Wales recently, during which Offspring#1 and I spent many happy hours beachcombing. In Lydstep Bay, while I was fossicking around at low tide after spongs and bryozoans and other good things, Offspring#1 was scaling the rocks high up in the splash zone and identified a series of scratch marks. Here they are.

I apologise for the absence of a scale bar: I reckon this field is about three centimetres by two, more or less. In my earlier post I suggested that they are scratches made by the radulae of questing molluscs as they scratch algae from the rock surface -- but I have never seen anything like this before, and  my postbag has hardly been bulging with comments on this either way. Now I've been on social media for a week or two, I thought I'd post this picture more widely to see if anyone had any ideas.