I have sat in that one (I recognised the list of experimental drops stencilled on the side) visiting a NASA Dryden pilot I knew from his Boscombe Down exchange tour. He said that it was effectively flown as a 2 crew (Pilot plus flight engineer) but to keep everyone happy any NASA pilot who was breathing was dragged out of the crewroom to sit in the right-hand seat and arm and push the 'drop' button on cue. Occasionally this resulted in slightly premature drops.....