The term 'EARLY ROCKET STAGE' refers to the Agena rocket stage, which was developed in the 1950s and 1960s by the Lockheed Corporation. It was used as an upper stage for a variety of launch vehicles, including the Atlas and Thor-Able rockets. The Agena was a two-stage rocket that provided additional thrust for payloads, allowing them to reach higher altitudes and greater speeds. It was also used to launch satellites and other spacecraft into orbit.