Green’s Dictionary of Slang

yenork n.

a crown, five shillings (25p).

1859		Hotten Dict. of Modern Sl. etc. 127: FLATCH-YENORK, half a crown. [Ibid.] 131: YENORK, a crown.
1873		N.S. Dodge ‘Vagrants and Vagrancy’ in Appleton’s Journal (N.Y.) 6 Sept. 308: Vagrant vernacular [...] is not remarkable for originality, and, unlike most low languages, possesses no spice of humor. The main principle upon which this peculiarity revolves, is to spell and pronounce the substantives in any sentence backward [...] Gin becomes shilling, and owt gins two shillings. Yenork is a crown, and deenop a pound.
1882		Sydney Sl. Dict. (2 edn) 11: Yenork - A crown.
1891		Exeter & Plymouth Gaz. 15 Oct. 6/4: The holder of the money gives each a ‘gen net’ and a ‘yenock,’ that is, 10s and 5s.

In phrases

half-yennork (n.) (also flatch-enorc, -yennork, -yenork, -ynork) [flatch n.]

half-a-crown, 2s 6d (12½p).

1856		H. Mayhew Great World of London I 6: Why, I’v cleared a flatch-enorc (half-a-crown) a’ready.
1859		Hotten Dict. of Modern Sl. etc. 127: FLATCH-YENORK, half a crown. [Ibid.] 131: YENORK, a crown.
1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor I 23/2: Flatch-yenork ... Half-crown.
1867		Berks. Chron. 7 Dec. 7/1: The Language of Costers. We’ve a slang, sir, and it is only know to ourselves. It puzzles the Irish and bothers the Jews [...] We call [...] half-a-crown a ‘flatchynork’.
1873		Sl. Dict.
1909		J. Ware Passing Eng. of the Victorian Era 149/2: Half-a-yennork (Com. London). Half-a-crown.
1935		R.T. Hopkins Life and Death at the Old Bailey 66: Flatch-ynork – Half-a-crown.