This challenge didn't take long. Our friends over at the Phillies Room knocked it out in 10 minutes. It took me a little longer. On the card Mantle has a distinctive patch on his left shoulder that is shaped like a shield.
I eventually found this photo:
I found this picture attached to an review of Jane Leavy's Mantle book, "The Last Boy" on Cleveland.com.
If you click on the picture it will enlarge and you can make out American League on the patch. From their I googled stuff like "American League Patch" and found this:
February - Dawgbones
January - Cards That Never Were