Green’s Dictionary of Slang

a sixpence.

1747		Life and Character of Moll King 11: Let me see, There’s a Grunter’s Gig, is a Si-Buxom; two Cat’s Heads, a Win.
1781		G. Parker View of Society II 56: You give him a shilling to buy a comb, for which he gives sixpence, so works you for another sye-buck.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1809		G. Andrewes Dict. Sl. and Cant.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1859		Hotten Dict. of Modern Sl. etc. 43: FYE-BUCK, a sixpence. Nearly obsolete.
1864, 1867, 1870		Hotten Sl. Dict. [as cit. 1859].
1874		Kent & Sussex Courier 9 Dec. 6/1: When next she meets him she had better swivel-eye him and pay him a chin-chin, and then perhaps he will Tip her a Fyebuck or Half a couter.
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’.
1895		Crowe Aus. Sl. Dict. 83: Syebuck, sixpence.
1901		Argus (Melbourne) 20 Sept. 6/4: The sixpence Ines many aliases, such as half a deaner, a syebuck, and a tester.
1907		Lancs. Eve. Post 26 Mar. 2/5: What is a tanner [...] syebuck.
1922		Dundee Eve. Teleg. 19 July 2/4: Sixpence is a popular coin in slangdom [...] ‘bandy’ [...] and ‘downer,’ ‘buck,’ and ‘fye-back’ [sic].