Originally, the back was handpolished to become "new." The worker could've finished it a micron thinner or thicker, as is done with each. In this particular case - ya got lucky - its a flat surface, and the scratch doesn't look too deep. I think I'd let them brush it out for you, using the same machine and setup as was done to make it "new" - and simply chalk it up to being within the variances of a handfinished product, even if there's a touch more to the story than you'd prefer. Good luck!