That was too cool.  I had to look it up.  
In an effort to control the crime inherent in illicit gambling, the Nevada State Prison in Carson City had the Carson City Mint make these coins to use in 'The Bull Pen Casino'.  An actual casino legally operated within the prison for the prisoners between 1932 and 1976.  Local politicians and even the Kiwanis club were regular visitors and gamblers at the legal prison casino.