Button is 2.5" with cardboard back ink stamped by the maker Greenduck, Chicago. Front of the piece is litho tin with an opening for a name card. Upper left has very shallow indent but the paint on this litho remains virtually Mint. This event began in 1923 to exhibit the latest oil industry technology and to provide a venue for oilmen to purchase equipment. In 1927 the exposition was opened when President Calvin Coolidge in the White House pressed a button causing a simulated gusher to blow wild on the exposition grounds. Given the construction of this button and its similarity to a 1928 Hoover campaign button (#327) we estimate this was used in 1928. Limited issue and the only example we've seen.