Green’s Dictionary of Slang

ten shillings (50p).

1859		Hotten Dict. of Modern Sl. etc. 127: GEN-NET, or net gens, ten shillings.
1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor I 23/2: Half-Couter, or Net-gen ... Half sovereign.
1873		Sl. Dict.
1882		Sydney Sl. Dict. (2 edn) 11: Net Gen - Ten Shillings.
1891		Exeter & Plymouth Gaz. 15 Oct. 6/4: The holder of the money gives each a ‘gen net’ and a ‘yenock,’ that is, 10s and 5s.
1935		R.T. Hopkins Life and Death at the Old Bailey 66: Net-gen – Half-sovereign.