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hankronThe fronts had a white coating applied before printing. So the front and backs are different.
They key is to be aware of the cardstock and edges of various cards (Old Judges to 1971 Topps), and notice when the online edge or corner is a strikingly inconsistant color.
For example, if there is an edge ding to a 1971 Topps Nolan Ryan that appears as a bright white spot in an eBay image, you will know that something is amiss. This is because the 71 Topps are printed on a dark stock and a ding to a genuine Ryan wouldn't appear as bright white.