This is a Significant Operational milestone.
The Type 173 was the first Britsh helicopter to employ two engines and twin rotors. It was also the first compound helicopter that transferred some lift from the rotors to a fixed wing. The pilot was Charles "Sox" Hosegood. Chief Designer was Raoul Hafner.
Source: AHS Calendar January 2012
Source: "Bristol 173 Takes the Air", FLIGHT, 11 January 1952