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If it were mine, I think I would have it authenticated for my peace of mind. I am not sure if the Chinese have started faking these yet, but I would rather be safe than sorry. As far as grade goes, it probably wouldn't matter much on a piece this scarce. (On the Pollock scale, a Rarity 5 is 31-75 pieces known to exist.) Trying to put a value on it, however, may be tricky. It depends on who is in the market for a piece like that at the time it is sold. It could sell for several hundred, or several thousand.