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Unless I've missed something, this issue is only occurring with the once-fired brass you purchased?

Do you know who fired this brass, what load was in it and what rifle it was?

Try the same load with your new Lapua brass and I think the problem will go away.

I know zilch about reloading, being a factory ammo man, but I do see marks like this from time to time on my brass after firing new factory ammo - particularly RWS. (2012, not the infamously "vivacious" 2013 batch)