1948-49 saw the first ever professional basketball card release, the NBA didn't exist yet, the card series documents one of the two leagues that merged to form the NBA.
These cards are very hard to find, or at least, they were back when I began and did most of my collecting in the days before the internet. It took me years to find any of them, and in a decade of collecting like my life depended on it, I was only able to hunt down roughly 5 cards. The George Mikan card is one of the most important basketball cards in existence, I have never seen one in person, last I knew (mid 2000s) it sold regularly for roughly $100,000. That's more than some houses cost in my area!