Green’s Dictionary of Slang

a sovereign.

1840		Courier (Hobart, Tas.) 27 Oct. 3/1: [advert, from UK source] [A] pair of fancy plod Drills, made very saucy, half a monarch and a bull .
1857		advert in ‘Ducange Anglicus’ Vulgar Tongue (1857) 45: Upper Benjamins built on a downy plan, a monarch to half a finnuff.
1891		Sporting Times 31 Jan. 1/4: We waited all the evening to see the purse of monarchs passed round.
1938		(con. 1900s) J.B. Booth Sporting Times 24: A couple of [...] Youths, the Elder of whom Asked him Politely if he had Dropped a Monarch.