The checklist from 2015 Topps Series 1 Baseball shows a Baseball History insert set. The 30-card set features just 15 numbers with an A or a B next to it. The reason for this is that the two cards make up one bigger card in a way.
Side A has a moment in history such as the start of the Geneva Conference or the Polio Vaccine being found safe or MLK’s March on Washington.
Side B has a baseball moment within a few days – or even on the same day – as the historical moment putting baseball into a historical context.
Consider the first cards in the set. Here is 1A – Geneva Conference Begins
You will notice no border on the right side of the card.
Here is 1B – Hank Aaron hitting his first MLB home run
This card has no border on the left side. When you put the two cards together, the design matches to create a bigger card putting these two moments in history together.