Model, the, n. — Green’s Dictionary of Slang

Model, the n.

[Pentonville was opened in 1842 and designed as a model prison on the ‘separate system’, i.e. continuous solitary confinement irrespective of one’s crime (pioneered in the Haviland Eastern Penitentiary in Philadelphia)]

Pentonville prison, Caledonian Road, London.

1856		H.Mayhew Great World of London 113: This is Pentonville Prison, vulgarly known as the model.
1890–1904		Farmer & Henley Sl. and Its Analogues.