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Dan Mathewson...and ask his opinion. Brian restores paintings and probably knows of an eraser type, or perhaps a solution (I'm just guessing here) that might work.
In any event, he will know your answer. Since this is a most valuable Christy Mathewson card, deserving of only the most careful and expert care, Brian (or some other professional) may even be willing to collect a fee to do it for you, to make sure it is done expertly -- sometimes, this really is preferred rather than taking a novice approach to a valuable card.
Brian posts here all the time, and his e-address should be readily available. He's usually quite helpful and really knows his stuff.