Mat,
I cannot see too clearly on my laptop screen, but the score marks seem to go very near the lip of the bore. IF this is the case then surely the plot thickens.
On the other hand if the circlip was missing or badly fitted and the gudgeon pin moved, AND assuming the the distance to the bore is greater than the width of the bearing in the piston then the gudgeon pin would be supported on only one side of the piston. This would cause the piston to twist and due to the enormous friction between the piston and the bore would cause a failure at the weakest point.