…before the advent of computers? And the balls too?
I’m not sure why you’re asking about the balls. Just paint 75 balls and drop them into the cage, right?
As for the cards themselves, that’s a better question. Google gives me this Voat thread that asks the same thing. Up to you what you think of the responses.
No one in that site seems to be sure. Today I played what is the second bingo game in my life. I’m 52.
Not the same question but relevant: How many possible Bingo cards are there? - Factual Questions - Straight Dope Message Board
I’m not sure whether the question is: How were the random numbers selected?, or Once selected, how were they printed?
For the latter question, I think there was a fixed number of possible cards (perhaps 600 originally, or 6000 in one case per the preceding post) and then those 600 (or 6000) cards were just printed over and over like the pages of a 600-page book. If this seems expensively tedious, realize the “bingo books” would have sold better than most books.
From first-hand experience I know that, even in the computer age, publishers sometimes do preparations that seem masochistically tedious.