Wildlife Fact File cards! You got a nifty 3 ring binder and then they would send you these cards in the mail as part of a subscription. I wish I still had mine!
I still have my original collection from the 1970s. Back then, they sent you a plastic green box (in those days, it was called "Illustrared Wildlife Treasury") to hold all your cards. That was before the 3-ring binder.