Cards not for Keypunch Use
If your deck of punched cards contained information critical to the national defense of the United States, you had to protect that information somehow. The solution provided by the government was a "cover sheet" in the form of a card that was to be placed at the head of each such deck of cards. This was in card format, but it was on stock thicker than a card. Therefore, the "cover sheet" had to be removed from the card deck prior to reading the deck, and added back later, after the deck was read.