Green’s Dictionary of Slang

bar of soap n.

drugs.

1940		J.G. Brandon Gang War 200: ‘Cargo of what, William?’ ‘Bars of soap,’ Big Bill answered tersely [...] ‘Dope.’.
1953		Anslinger & Tompkins Traffic In Narcotics 303: bars of soap. Drugs.
1960		J. Franklyn Dict. of Rhy. Sl.
1992		R. Puxley Cockney Rabbit.
2002		B. Kirkpatrick Wicked Cockney Rhy. Sl.