Green’s Dictionary of Slang

(Aus.) a sovereign (£1).

1919		W.H. Downing Digger Dialects 14: bucking horse (n.) — Sovereign (coin).
1924		(con. WWI) A.G. Pretty Gloss. of Sl. [...] in the A.I.F. 1921–1924 (rev. t/s) n.p.: bucking-horse. Sovereign (coin). Derived from the impression of St George and the Dragon.