Green’s Dictionary of Slang

20, in a variety of contexts, e.g. 20 years’ prison, a 20-ounce packet of tobacco, $20, £20 etc.

1823		‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc. 155: ‘Set off at score,’ a road phrase for a horse gone off full tilt, perhaps at 20 miles an hour.
1855		Sydney Morn. Herald 12 May 4/4: [H]e offered witness ‘a score’ to allow him to ‘work the jug’ without molestation [...] it was an offer of £20 to be allowed to pick pockets at the Commercial Bank.
1876		C. Hindley Life and Adventures of a Cheap Jack 39: Now if I had any friend that would lend me a score or a fifty, it would be the means of getting myself in my old position again.
1885		Bulletin (Sydney) 14 Mar. 10/3: And hoarsely do the people roar – / ‘This is the kid what gave his “score”’ [i.e. £20].
1899		A. Binstead Houndsditch Day by Day 104: A cain-an’-abel that cost three-score.
1904		Sun. Times (Perth) 21 Feb. 1/1: What happened while mother was lighting the fire cost him a score to smooth over.
1906		A.H. Lewis Confessions of a Detective 15: I was in the instructive company of a score of these.
1929		Hostetter & Beesley It’s a Racket! 237: score—Twenty dollar bill, or units thereof—hundred, two hundred, etc.
1936		J. Curtis Gilt Kid 20: I earned myself a score. And twenty pound’s very handy what with things the way they are.
1946		Singleton Argus (NSW) 4/2: ‘Listen, there is another score in it when I see you in Sydney; don’t be a — mug; take the spin now and the rest after.
1958		F. Norman Bang To Rights 153: The magestrate asked me where I got the score and I told him I had won it.
1962		R. Cook Crust on its Uppers 35: In the end she peeled off a score.
1966		J. Holledge Great Aust. Gamble 105: After failing to borrow a ‘score’ from me he departed.
1977		G.F. Newman Villain’s Tale 20: ‘Gonna stand you in about a score, son,’ the player said [...] He counted twenty pounds and added it to the pot.
1981		J. Sullivan ‘Christmas Crackers’ Only Fools and Horses [TV script] There’s a score for yer.
1996		J. Murphy A Picture of Paradise in McGuinness Dazzling Dark (1996) Act I: I’d expect to be paid for me work. A fee like [...] A score a week?
1996		J. Byrell Lairs, Urgers & Coat-Tuggers 223: ‘Needless to say, if he’d offered me a score I’d have been on top of the world.’.
2000		Guardian 21 Jan. 32: I slipped him a score to stick with me.