Hi Simmon,
According to your pictures, it seem that this dammage could be a piston over heating result. It can be the fact of an optimist ignition point or a too lean air fuel ratio. I think it appears on the hotest cylinder (no 5). This hypothesis could be confirmed by the piston top aspect and the combustion chamber. If it is light grey, it can be an over heating. Paradoxically, forged pistons (as I believe you have) are more sensible to the thermal variations (due to the alloy expand coefficient) than hyper pathetic piston (that are hyper siliced). An other point is the
oil quality. Remember that for the first mileages with a new engine we have to use only mineral
oil (no
synthetic oil). Hi hope my humble point of view can help you.
Bernard