I've seen SC pistons chunk the edge of the piston off from detonation. Yous looks like it wore off rather than breaking off. The edges look smooth where the piston is missing. Or at least it appears that way. I wouldn't think and NA would detonate enough to chunk a piston. I guess pull it out and get a closer look.
While diagnosing a very rough idle I found this:
Code 43-knock sensor bad
Fuel injectors were were badly clogged with rusty bits from a fuel filter that had come apart, especially number 3.
I was leaning toward detonation damage myself. :frown: