The card fronts showed an elongated image of the player, with his last name, team, and league superimposed above. This set is unnumbered. The ACC designation is T201.
The card backs were broken into two units, upside-down to one another: the bottom 60% gave the names and basic biographical data of both players on the card, with an ad for Mecca Cigarettes at the bottom; at the top was the top 40% of a second baseball card image similar to the top of the other one, so that when it was folded over the 2nd card image 'borrowed' the legs and card bottom of the first player image.
So, each card in this set features two players, one front and one back.
This technique would be resurrected for the 1955 Topps Double Headers issue (probably suggested by Topps employee Woody Gelman, who almost certainly knew of this earlier card issue).
Prominent cards in this set include:
Albert Bridwell / Christy Matthewson
Ty Cobb / Sam Crawford
Walter Johnson / Charles Street
Earl Gardner / Tris Speaker
Fred Falkenberg / Napoleon Lajoie