Here is an eBay auction of one rare cap.
Imagine. It's 1955 and you are collecting the Red Man tobacco set of Baseball cards. You finally have amassed 50 cards and realize you have enough for a special prize. Check out the below picture of the back of one of the cards.
With 50 tabs cut from the bottom of the card you can receive a big league cap of your favorite team. You send them in and receive the Brooklyn Dodger cap seen here.
I have no idea how rare this is, but I can't believe many still exist; especially in this condition. It still retains a Redman Baseball Cap stamp on the inside band and looks like it has never been worn.