Here is a link to the OldCardboard.com webpage which identifies Cobb's "Top 5 Rookie Cards":
http://www.oldcardboard.com/ref/rook...tail.asp?id=64
Jeff's Detroit Team Postcard pre-dates all five true Rookie Cards (Individual Appearances) and here is the 1905 W601 Detroit Premium which is the earliest image of Cobb on a ML collectible: