Lower playfield between flippers
Here's the worst of it. There is signifcant ball wear down to the substrate of the lower layers of ink. The entire bonus area was painted and covered with mylar.
Over time lacquer develops cracks from ball impact. These tiny cracks get carbon dust inside them from the arcing of the solenoids being fired. If a playfield is not properly mainted and waxed often, more and more dirt will get in and cause the cracks to widen, as in what we have above.
When the center section was touched up, it was not leveled - that's why it looks a bit lumpy.