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RhysLat year a Wolverine News postcard was found on ebay with the earliest postmark(I think it was 1906, or early 1907) which means that the issue was actually issued in Cobb's rookie year as opposed to how it had been catalogued up to that point. It may not be considered a "card" by some people but a postmark is pretty good evidence as to when something was issued.
This pre-dates the Dietche postcard which was issued a bit later.
I would consider this to be Cobb's first "card".