Green’s Dictionary of Slang

honest; also as n., an honest person.

1959		J. Gosling Ghost Squad 25: Thieves’ argot, spoken properly, is a foreign language which needs to be learned [...] ‘six and eight’ means ‘straight’ or honest.
1962		R. Cook Crust on its Uppers 41: Not six-and-eight exactly, but settled down very cosy.
1984		Partridge DSUE (8th edn) 1074/2: since ca. 1930.
2000		Sun. Times News Rev. 12 Mar. 3: He was the archetypal ‘six and eight’ – the prisoner who would go straight.