ShyTorque
The point is that the system existed and could easily have been loaned at the time for particular exercises being easily transferable between these aircraft (there was a 47-D stooging around at low level in NI at the time of ZD576’s departure from whence the equipment could have been had).
From a navigation systems point of view, the use of such (with a misleading fudge) fits what happened.
But what evidence is there if the hardware was removed from the wreckage?
If, say, they had been pressured by someone (MI6?) to tighten up the approach for security reasons by using the Americans’ gismo, this would have been a bit unusual; these officers were able professionals who would have paid attention to detail - when presented with an unusual procedure they may have asked if a particular SSR code was applicable – caught on the hop, Americans can be refreshingly open (eg the “looking for their equipment" bit) and one may have suggested one that they used. If 7760 was set before the impact (as discussed before, the idea of such dial switches being so far removed from the expected usual by “soft object” impacts is unlikely and the attempted change by the crew in an urgent situation not normally recommended or likely in these circumstances), as can be confirmed or not from the radar recordings (which no interested party seems willing or able to pursue), this is the possible evidence.
Do not waste their effort – they had the wit to tell us what they were doing.