It could be wear from a damaged pitted ball. Would bet the same ball has been in the game since new. The playfield has never been polished/waxed. Look at the ball arch at the top it appears to have a ball groove in it. If it was dust, dust would be every where, no gap around the flippers and lower plastics.

Here is a photo of my Flying High, lots of wear, and dried out paint and finish. Looks similar to the OPs photo.
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Here is the photo after cleaning and a medium effort in polishing, then waxing the playfield. This fall, I might try a more aggressive polish. This time I was being over careful not to remove to much paint surface.

Zoom on the photo, the wear is still there, and with many chips in the blue section.
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