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Hello, I'm fixing an old banjo guitar.
Into the resonator there is kinda like "mark" on the wood.
Maybe with a little oil will gain a bit of color. Unluckly has that humidity spot.
Never seen one, do you?
Here are some hints for handling moisture spots on wood. Don't know which will work best in your case.
Here's one a little more contemporary in construction that says its design is laser-etched. I favor your design more.