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Certain props are sometimes attached to the set
when it's built. Rather than being removed everyday
when the shooting day is over, they're left in
place. These are known as set-dressing props & are
considered to be part of the permanent set.

Near every hatchway in Serenity is an emergency breathing mask. I assume their purpose it to provide
breathing air in the event of an 'Atmo' failure. There were many of these spread throughout the Ship set
near the hatches, in Inara's shuttle, in the cargo bay, on the bridge, all over. They appear in every episode.

They date from the late 60's/early 70's and are high altitude breathing masks for use by crews of military aircraft.

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