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identify7A postcard with a baseball theme becomes a baseball card at the point that the original purchaser makes their choice (rather than "Sunning in Miami, wish you were here)" based on the theme portrayed.
As another example, in the early '50s topps produced non-sport cards depicting cars (Wheels), airplanes (Wings), trains (Rails) and sailing boats (Sails); but no warships. I cut out the side of a plastic, glue together warship model box cover which showed a warship, and the resultant cutout was about the size of the Topps cards. This cutout survived with some of the Topps, and other cards of my youth.
The point being: the identification of a baseball (or other similar) card, as such, is made on a gut level by the original owner of the item. This identification, although irrefutable, can be difficult to ascertain. imho.
To me, my cutout warship is a legitimate card, eventhough that was not its manufacturer's intent. I may have some difficulty selling it, but why would I want to do that anyway.
Sorry for the rambling, all. I guess this doesn't really clear up Hal's inquiry.