Green’s Dictionary of Slang

1. a halfpenny.

1859		Hotten Dict. of Modern Sl. etc.
1864, 1867, 1870		Hotten Sl. Dict.
1881		Morn. Bulletin (Rockhampton, Qld) 18 July 2/6: A half-penny [...] may find the following; ‘bawbees,’ ‘browns,’ ‘camden town,’ ‘coppers,’ ‘ flatch,’ ‘gray,’ ‘madge.’ ‘make,’ ‘mag or maga,’ ‘posh,’ and ‘rap’.
1972		Dodson & Saczek Dict. of Cockney Rhy. Sl. 24: Camden Town. Brown [...] (up to c.1914; widely used in cockney, but now only by buskers and street traders).

2. heroin.

2011

J.J. Connolly Viva La Madness 41: ‘Brown?’ I ask. Heroin [...] ‘Oh yes. the Camden Town’.