Right, first things first, a 4 point seizure is NOT caused by lean mixture.

It may well be avoided by an overly rich mixture (causing cool running) but the root cause is insufficient clearance, either due to; thrashing from cold, running in condtions so cold the barrel never gets warm (I did this at Brands last year) or bad matching of components at assemby.

Those marks do look quite clear (good pic by the way) but I've seen plenty of barrels come off perfectly working engines and have similar marks on the piston.
I'm not saying it's ideal as it's clearly not, but if it's been running for years in this setup.....
_________________
Please do not PM me technical questions, if you can't find it on the Forum start a thread