1957 ... ventilation plug!

... fitted to a prototype blast shelter, at a Nevada test site, a large hollow steel ball will temporarily block a ventilation duct during an atomic explosion. The huge pressures and winds of a close-by detonation could travel through the ventilation system of a shelter and kill or injure the occupants. Any sudden change in air pressure will instantly pull this big steel ping-pong ball  into and choke the ventilation opening.

... judging from the 'scouring' pattern it may have already gone through a test at close range.

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