It would show more tapering if it had been drilled and the sides would be smooth and not textured like that... looks more like a break of some kind to me... far from an expert but I have drilled a LOT of holes in metal :)
After looking at it some more I agree. The ridge is not tapered, the field inside is flat. I wish I had a microscope camera in this case. I have seen smeared letters before do some funky things...maybe this is a case of that...dunno.