The three "o" image Kaiji saw from the previous chapters is actually another switch that you need to poke, except with 3 holes. It hids another compartment which has assortment of other types of cards, including 2.
I should clarify that Kaiji didn't see the content of the compartment since he couldn't open it, but he assumed it has cards that are needed to win completely, including the Ace killer, 2.