The life-preserver plays a major part in Anthony Trollope’s novel Phineas Finn, first published as a monthly serial from October 1867 to May 1868. Finn, an up-and-coming young Irish politician, habitually carries a life-preserver in an inside coat pocket for self-defence against robbers when walking around London at night. One day on the steps of Parliament he unwisely takes it out and brandishes it for emphasis while inveighing against a political rival, which gets him in no end of trouble when said rival is shortly afterwards found bludgeoned in a London street.
When the BBC serialised Trollope’s Palliser novels in 1974, the said life-preserver was shown as a cudgel with a flexible neck and a round head; I can’t say how well researched that was but the Beeb are usually fairly scrupulous about that sort of stuff. Here are some original 19th-century examples: