Yep ,nice one ! There are glazed and unglazed ones ,yours look like unglazed .I think that spiral pattern is not common . Most are complete circles not spirals ,like these.
I would mix small amount of Bar Keepers Friend or other mild cleanser with water to make a paste .Use Q -tip and rub paste in a area near the end of painted spiral to test .[]
Some were painted with or without clear glaze then fired ,some were painted after firing .The ones i have found with out clear glazing seem to be faded badly by soil .
There have been rash of fake painted Chinas on e-bay ,so maybe the stains are good proof of age if selling .
For more info you might try posting on marble web site like _Land of Marbles _Marble Connection ,or _ All About Marbles .
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