On the other hand, The U.S. Department of the Treasury has maintained the Bullion Depository on the post since 1937. It is a fortified vault building (in the postcard) located adjacent to Fort Knox, used to store a large portion of U.S. official gold reserves (4,578 metric tons of gold bullion, i.e. roughly 3% of all the gold ever refined throughout human history) and occasionally other precious items belonging or entrusted to the federal government. Even so, it is second in the U.S. to the Federal Reserve Bank of New York's underground vault in Manhattan, which holds 7,000 metric tons.
About the stamp
The stamp is a Global Forever First-Class Mail International one ($1.10), about which I wrote here.
Fort Knox - Wikipedia
Fort Knox - Official website
sender: The Crouchers (direct swap)
sent from Carrollton (Kentucky / United States), on 04.12.2013