What causes those little C shapes near the sling?

I believe those are a classic example of ball swirl. Generally caused by a dirty pf combined with the rolling/spinning action of a pinball. I tend to see them on older games with lighter areas of the pf. My Black Knight was so full of them, they blended into a kind of pleasant-looking patina. On a modern game like ST, that has a clear coat, you could probably buff or rub them out if you were careful.