Keystone, MissouriKeystone is an extinct hamlet in northeastern Clinton County, in the U.S. state of Missouri.HistoryA post office at Keystone Station of the Leavenworth branch of the Chicago, Rock Island, and Pacific Railroad was established in 1874 and closed in 1919. A nearby settlement known as West Keystone existed as well. The name of Keystone is survived by a lone county road.