This is a Notable Operational milestone.
This was the XH-51A fitted with awing and a turbojet mounted on the port wing. It achieved a speed of 302.6 mph/487 km/h. The furst flight was conducted by Don Segner. Fotr this initial flight the turbojet was not used. The first flight when the jet engine was igited took place on Apr 10, 1965.
Source: "Rigid Rotor Rewind: Lockheed Helicopters Come Full Circle", Raymond L. Robb, VERTIFLITE March/April 2017, Page 51