Green’s Dictionary of Slang

1. (US) a nickel, five cents.

1859		Matsell Vocabulum.
1881		Trumble Sl. Dict. (1890).

2. a sixpence (2½p.).

1839		H. Brandon Dict. of the Flash or Cant Lang. 162/2: Downer – a sixpence.
1857		J. Archbold Magistrate’s Assistant (3rd edn) 444: Sixpence, downer, also sprat.
1860		Hotten Dict. of Modern Sl. etc. (2nd edn).
1873		Sl. Dict.
1881		Morn. Bulletin (Rockhampton, Qld) 18 July 2/6: For our next coin in value twenty names are found, viz: - ‘sixpence,’ ‘bandy,’ ‘broder,’ ‘cripple.’ ‘downer,’ ‘fiddler.’ ‘fyebuck,’ ‘half-hog,’ ‘kick,’ ‘lord of the manor,’ ‘pig,’ ‘pot,’ ‘say saltee,' ’sprat,’ ‘snid,’ ‘simon,’ ‘sow's baby,’ ’tanner,’ tester,’ and ‘tizzy’.
1885		Household Words 20 June 155: Two more names for a sixpence are a downer and a ‘tanner’ [F&H].
1922		Dundee Eve. Teleg. 19 July 2/4: Sixpence is a popular coin in slangdom [...] ‘bandy’ [...] and ‘downer,’ ‘buck,’ and ‘fye-back’.