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WesleyYou are going to get a lot of varying opinions on this one. My answer is almost the directly the opposite of Brian's answer. Unlike modern baseball cards which are mostly found in wax or foil packs, prewar baseball cards were distributed in many different ways and one of those ways was as postcards. Postcards can certainly be considered baseball cards.
I also collect Ty Cobb cards and I definitely think that Cobb RC are his postcard issues. If you don't consider postcards to be cards then the first Cobb card would be either the T206, E90-1, E92, E101 or E102. All of these, however, have issue dates at least two years after the Dietsche, Wolveries News, HM Taylor, and Novelty Cutlery postcards as they all have 1907 as the initial issue date.
Likewise for Lou Gehrig, some consider his 1927 York Caramel Type 2 his rookie card. This is ignoring several exhibits that predate that card including the 1925 Exhibit with postcard back which I believe most of the hobby recognize as Gehrig's rookie card.