BOAC, are you suggesting the wife was or had been in the fourth seat?
---And that's why the fourth seat was retrieved, if only to check the straps?
This was a flight crew that after the SELCAL check with ATLANTICO, apparently switched immediately to the DAKAR frequency. And a flight crew that left the radar in CAL mode until 0208.
The Captain had asked the PF whether he wanted to take his rest and the PF said he wasn't sleepy. The Captain apparently then took his rest during the period that the PF would have been resting.
Is the PF unbelted because that facilitates his turning around if he wants to talk to someone behind him?
All that said, its hard to believe the presence of a fourth person on the flight deck would not have been hinted at by now, through some leak to the media.