Mark Lewis introduced the idea of collecting the first year card of players in his CPU Card Prices Update magazine in 1979-1980. The idea flourished. There was no stipulation as to size, shape or material of a rookie card; or whether the card should be a solo picture of a player or to picture him surrounded by team mates.
My Rookie card: The first publication of a card with a picture of an unaccompanied player.
Branch Rickey: 1906 SL postcard (accepted) as his rookie card.
But Rickey's first card is the 1907 W600. None are known to exist. But it is "his" rookie card - there is no substitute.
Note: The Rose Company commenced distribution of all its first run postcards - 12 players from each of 16 teams - in August, 1908. Subsequent postcards were published of different players and backup postcards of players from the first run.
All these later produced postcards were printed with only the player's names - no associated team was added - no team postcards.