I know an ex BOAC 707 Captain, now aged 90 and put the question to him. He has just checked one of his many logbooks. Incidentally he achieved a speed record in the early 1960s flying LAX-LHR in 9hrs 15 minutes, (airborne not chock to chock). He says it might have been SF0-LHR though.
Back to the question. He operated BA 591 LHR-LAX on Jun 3rd 1962 in B707 G-APFG. It went via JFK which is where he got off. Strangely his next sector was from JFK-Kingston. He reckons he must have been supernumary on the BA591 as he has the Captain's name down as Buxton and not himself.