I believe the supernumerary (stupid name) is only found on wide-body aircraft. It is a small area right behind the cockpit where extra jump seats (or possibly sleeping quarters are located).
On the 737, there is a galley, flight attendant seating area, and a lav behind the cockpit. Those would not be considered as part of a supernumerary. On cargo aircraft..... the shipping containers fill that space.
On the 767, all supernumerary seats have the standard "box mounted" oxygen masks with the inflatable straps. These are the same units used by the pilots.
The only image I have is:
The FedEx aircraft that I worked on had full-face masks that covered both the mouth and eyes. I've only seen these on FedEx aircraft. They were not available in the jump-seat positions.