Thanks Versuch – I will talk to the local glass people this week.

Did a bit of digging on Limpets and they have a home site that they graze and return to – as they grow their shells also grow and abrade the surface as they are likely to be in contact with it daily – which is why there are the concentric rings and the foot mark in the middle.

The orange stuff around the edge of the glass seems to be degraded paint as it came away with a wipe of solvent.