Well in the mid 1870s in the then British Colony of Victoria, (Australia), the Local Government Law addressed the question 'What is a Plumper?' Here is the newspaper report:
https://trove.nla.gov.au/newspaper/article/244267370 The Herald 22 November 1876.
Earlier, back in August 1870, in the then British Colony of Western Australia, there's plumper matters over vote splitting ... here:
https://trove.nla.gov.au/newspaper/article/3756117 Perth Gazette 19 August 1870.
Back in the 1820s in the then British Colony of New South Wales, there's mention of plumper as though it is an alternative word for palter
https://trove.nla.gov.au/newspaper/article/2186608 Sydney Gazette 27 Sept 1826
And yet, in the late 1820s, in the then British Colony of Van Diemen's Land there's a plumper being quadruple votes ...
https://trove.nla.gov.au/newspaper/article/4211827 Hobart Town Courier 5 December 1829
And then I googled :
https://www.wordnik.com/words/plumper noun In elections, a vote given to one candidate only, when two or more are to be elected, thus giving him or her the advantage over the others; a person who gives his or her vote thus is said to plump, or to plump his or her vote.
noun A voter who plumps his or her vote.
noun A downright, unqualified lie.https://penandpension.com/2015/09/30/straights-splitters-and-plumpers/ ".....
almost every constituency, large or small, returned two members to the House of Commons. That meant each elector had two votes. Eighteenth-century electors had all the modern tricks available to them: voting for a candidate you were not expected to favour, changing your mind at the last moment, or voting for someone purely to deny another candidate a majority. ......linked to the various ways you could apportion your two votes between the candidate. And that was without the extra bonus of demanding a large bribe for voting as someone wished you to do. ...."JM
