The story I heard about the Flight Engineer's panel and the gap that opened up at cruise involved the last supersonic flight of each airframe. I believe that Concorde had the last Flight Engineers in BA; so to mark the passing of the aircraft and their own demise as aircrew, the F/Es put their hats into this gap at cruise....as the airframe cooled and the gap closed up, the hats were trapped forever!
Watching Concorde start was always entertaining..the two large diesel air compressors, one for each side, roaring away and belching exhaust smoke...