It's a master hub.

The others are pressed with this one to make the reverse dies (hubs) for actually pressing the coins. I seem to remember this from some book on making US coins, or was it a movie in grade school?

Short version:

The master hub is then heat treated to make it hard. The master hub is then used to make only a few master dies via hubbing, which involves pressing the master hub into a steel blank to impress the image into the die. The master die is then used to form as many working hubs as needed through the same process, and then the working hubs are put through the same process to form the working dies. These working dies are the actual dies which will strike coins. The process of transferring the hub to the die can be repeated as many times as necessary in order to form the number of dies needed to make the amount of coins required. The difference between a hub and a die is that the hub has a raised image and a die has an incuse image, so one forms the other.

Further Reading Resources:

http://coins.about.com/od/coinsglossary/g/coin_hubbing.htm

http://en.wikipedia.org/wiki/Coining_%28mint%29