After BEA Flight 548 incident in June 1972, all British-registered airliners have mandatory cockpit voice recorders. The TV series Mayday claimed no voice recorders were installed in Hawker Tridents before the report of the incident was released.
9 months before BEA Flight 548, a Chinese Air Force Trident 1E (AP-ATL) crashed in Mongolia. Lin Biao, the second-in-charge in China, was killed as well as his family members in the incident.
The incident itself is still mysterious, but some source claimed that there were voice recorders in the cockpit and the chat inside the plane was recorded. Many sources cited that the Chinese officers did not know the black box was installed, so the voice recorder was only installed by either Pakistan International Airlines (previous owner of the airliner) or by the manufacturer.
The question is : Were there any Hawker Trident installed cockpit voice recorder before BEA Flight 548 incident?