Thanks rog747, the announcement said that they were the first passengers on the plane as it had just been delivered and that it was a Boeing. The recollection of silver on the fueselage could well have been the lower part as they'd have boarded by steps from the ground and (for them) the 707 would be huge after the prop job they flew over the Atlantic (via Prestwick). I suggested Northwest, but couldn't remember if they had much red in their livery back then.