In post # 50 above I mentioned the "black box" from which the flight path of Hotel Golf was reconstructed after the Stockport crash, and in post # 52 "forget" wrote that the Trident was the first commercial aircraft in which such a device was installed.
I think the Trident's contribution to 'black boxes' followed the Staines tragedy. The AAIB (or the then equivalent) called for mandatory CVRs following that accident, as cockpit voice information may have helped ascertain why the droops were retracted, causing the stall and crash.