[they were] hit with a life preserver or some such thing on the back of the head.
Agatha Christie. And Then There Were None. p.99 (1939)
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At first, of course, he denied everything. But when I gave him every particular that had occurred, he tried to bluster and took down a life-preserver from the wall. I knew my man, however, and I clapped a pistol to his head before he could strike.
Arthur Conan Doyle. Sherlock Holmes: The Adventure of the Beryl Coronet
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A passing thought occurred to me that Miss Murdstone, like the pocket instrument called a life-preserver, was not so much designed for purposes of protection as of assault.