Huh. It kind of looks like flow lines, but I'd expect a lot more deterioration in the devices around the perimeter if the die were that worn. Odd...
In photo #5 the darker lines look to be a bit incuse into the surface. I would suspect that the darker lines are caused by being struck through grease / debris which scoured the surface in the direction of metal flow when being struck. Note lines on reverse only in lowest parts of design where metal has the most flow/ movement. on horizon and in the river.
