The X-1 was built and reportedly some tethered flight teste were carried out. Some sources claim there may have been a connection with the Fairchild company
and that the X-1 may have carried the Fairchild M-257 or M-258 designation (speculative!).
Please see attached photo and text.
2/4-seat experimental S/VTOL
two 340hp Lycoming GSO-480 piston engines
wingspan 24ft; length 33ft; height 6.917ft DETAILS: The X-1 (M-257 or M-258) Aero-VTOL was an experimental S/VTOL aircraft developed by the Robertson Aircraft Corporation (formed in October 1956 as a division of Aero Design, Inc). The aircraft also appeared in 1956 and in its construction a modified Aero Commander 500 fuselage and many other parts and components were used. The X-1 used the so-called propeller slipstream deflection principle and underwent only very limited testing and reportedly made its first (tethered) flight on 8 January 1957. However, further development was soon halted. The aircraft may have been referred to as the X-1 (Roberton) or M-257/M-258 (Fairchild) and it had two 340hp Lycoming GSO-480 engines and the wings were equipped with large full-span t/e flaps providing the S/VTOL characteristics. The wingtip tanks also served as endplates.