This is a Significant Operational milestone.
The Bell 533 HPH (High Performance Helicopter) was a compound research helicopter. The speed was achieved in level flight with the 2-blade main rotor configuration. This is an unofficial speed record that remains unbeaten.
The sole example of the Model 533 is displayed outside the main building of the U.S. Army’s Aviation Applied Technology Directorate (AATD) at Ft. Eustis, Virginia
Source: "Hybrid Helicopters: Compounding the Quest for Speed", Vertiflite, Summer 2006, page 44