That is a really interesting picture. I can't imagine a valve hitting the SIDE of a valve relief in the piston (those angled depressions are called valve reliefs). The valve should be opening and closing at the same angle as that valve relief and if the valve were ever to strike the piston, it should strike with the head of the valve flat in that relief. Notice your push rod tubes. They are at the same angle as the valves and the relief. It should not be possible for the edge of the valve to strike the side of the valve relief. If the valve made that mark, it would almost certainly be bent badly enough that the engine would not run. Since you have the engine this near disassembled, my advice would be to take that valve out of the head and roll it on a piece of glass. If it is bent, you will be able to see it. But if it is bent that badly, the engine should not run. As Yul Brenner said, this is a puzzlement.