This is the back of a Chicago World’s Fair postcard, and it has a Century of Progress postmark and stamp. The 1 cent stamp shows Fort Dearborn, has 1833 in the upper left and 1933 in the upper right. There were three different stamps issued for the fair by the U.S. Post Office, and this is the most common one. It is on at least 30% of my world’s fair collection.
The postmark has the dates the fair was open in 1934- June 1-November 11. I feel like this postcard is the ultimate world’s fair postcard because it is a card of the fair, has the world’s fair stamp, and world’s fair postmark.