In some pics it looks painted, in others it doesn't. Going by the obvious changes in the sight greeblie, these pictures were obviously taken at during junctions before, during, and after production. But, that Mystery Disc is surely a mystery no longer - it is milled. And the center dark spot is not a hole, but a well placed divot with some dark something in it.
Each one of those scars in the disc are most likely where there was deep rust. Perhaps because the Disc is bare metal, it somehow or another began to rush, so they give it a quick polish with some sandpaper or something. Those deep pits refused to polish out flat, and thus they remain as deep pits in the surface metal - I've seen this on several antique katana that I own, prior to polishing and restoration, of course.