Bjorn, I admit I don't know much about these old jetons....or new ones for that matter. I'm not so sure it's an error. Both sides are serial numbered in the same manner. If the little extra curl, top and bottom, is part of the design the makers might have considered that the best place to put the number. Have you seen other 500 chips from that series that are treated differently?
I may keep the chip, as least for now.