I haven't heard anything about opc gray backs but since nobody else has responded I will share that opc typically used a white card stock similar to what factory topps traded cards used. Topps used the gray card stock for their base sets. My guess is that if opc gray backs do exist it would have been some kind of rare mix up during the printing process and the numbers on such a variation would probably be extremely low.
Where and what have you heard on the topic? If you read an article can you share a link?
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COLLECTING BROOKLYN DODGERS & SUPERBAS
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