I have received the infos I was looking for!
So the observer seat is now equiped with seatbelt. In fact, seatbelt was the same than the one of the pilot.
For the instrument panel, not sure it was left in aluminium - but not sure it was overpainted in gray!
So, I will work logically: if the interior was overpainted in grey to avoid to the crew to be blinded by sun reflexion, it's logical tio paint the instrument panel in grey...
Little progress: mr Surfacer sanded; the little gap between the top and bottom parts of the wing filled with Perfect Putty (you apply putty, use a q-tip with water and that's all...) and, of course, observer seat completed