These cards are really associated with 1968 Sports Illustrated posters, and have nothing to do with cereal or Corn Flakes.

The numbering on the front of the blank-backed cards are the same as the SI posters.

The belief is that they were used as part of the retailer's point-of-sale display rack. It would tell the consumer where in the rack below the associated player poster is located. They were made to be housed in the corner of the poster rack that you would peruse.

You will notice that each team is given a number, which is the column within the rack of where the poster is located. "N" is for NFC and "A" for AFC. They then used the player's uniform number.