Green’s Dictionary of Slang

(US drugs) a mixture of morphine and cocaine, usu. in proportions of 4:1, the equivalent of the heroin and cocaine speedball n.¹

1928		Orlando Eve. Star (FL) 4 Mar. 27/4: He had kept a pure cache of ‘ and C’ for his own use.
1930		‘Dean Stiff’ Milk and Honey Route 209: M. & C. – ‘Marmon and Cadillac,’ meaning morphine and cocaine.
1953		Anslinger & Tompkins Traffic In Narcotics 312: M. and C. Marmon and Cadillac: a mixture of morphine and cocaine.
1969		R.R. Lingeman Drugs from A to Z (1970).