Bruce, I appreciate your clear explanation of the markings.
My impression, as opposed to knowledge, is that the browning is a change or loss of black pigmentation from the original material and not a reversible material change. A dye that saturates the material rather than painted on would seem to not be merely a cover.
I'll stop here to let someone with actual experience chime in on a solution.
Please let us know your solution when you find it.
Erick
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