I've several guns that exhibit this feature and have seen many more. I had assumed scratches, but had trouble reconciling just how all these guns got the same stratches. I'd toyed with the idea of some common "brushing" technique, but the engraving did not bear this out. So, I'm delighted for a sensible explaination. I'll have a look under a microscope and see if I find anything worthy of reporting.

DDA