Green’s Dictionary of Slang

(drugs) Durophet.

1967		Glatt et al. Drug Scene in Grt Britain 115: Minstrel (black and white) – Durophet.
1969		R.R. Lingeman Drugs from A to Z (1970) 47: black and tan, black and white, black and white minstrel (from the colour) durophet capsules, 12.5 milligrams. [Ibid.] 191: nigger minstrel [...] durophet capusles.
1971		E.E. Landy Underground Dict. (1972) 34: black and white minstrel. [...] 12.5 mg. capsule of an amphetamine and a sedative.
1980		S. McConville ‘Prison Language’ in Michaels & Ricks (1980) 526: Amphetamine-barbiturate mixtures seem to have spawned a particularly vivid range of nicknames and images, often arising from the appearance or color of capsules in which they are taken. These include [...] black and white minstrel, [...] nigger minstrel, [etc.].
1989		J. Morton Lowspeak 104: Nigger minstrel – Durophet capsule.