Damage to cylinder #2 wall. Zooming in to the photo, it looks like lots of superimposed little impact marks rather than a scratch.
forumosauploads-12829.kxcdn.com/original/3X/1/c/1c...g
https://forumosauploads-12829.kxcdn....2_666x500.jpeg
My best guess was broken end of a piston ring dancing around, but I have now taken the piston out and so far can't see anything wrong with it or the rings.
I'll clean it up, check the ring gaps, and maybe smooth off the ends a bit, though I can't see any burrs.
Havn't decided whether to hone the cylinder yet. Probably not.
I suppose this damage could have been from a hard object that has since departed
Alternatively, could it be detonation or pre-ignition forcing impact of the ring ends onto the cylinder wall?
(The top ring gap was probably roughly in the right orientation on removal)
When I first got the car it seemed to have some pre-ignition on idle and had some run-on after switch off, both of which went away after a water decoke.