The Kellogg's cards came with a product called "Danish Go-Rounds".
It was basically a Pop-Tart but in tubular form with more filling. Here is the box back:
I know what they were because as a fat boy of 7 I scarfed down quite a few of them and had a partial set of the cards.
Now, as far as the reused set, I suspect that the company that made them had a contractual right to reuse them after a year or two.