Green’s Dictionary of Slang

a coat, a waistcoat.

1846		Swell’s Night Guide 127/1: Oliver widdles, the moon shines. Oliver sneaks, the moon hid under a cloud, has got his upper Ben on.
1876		C. Hindley Life and Adventures of a Cheap Jack 253: In offering these ‘Bens,’ the plan was to put them on to show how well they fitted.
1887		Daily Trib. (Bismarck, ND) 23 Oct. 4/1: A coat is a ‘topper;’ an overcoat is a ‘ben.’.
1914		Jackson & Hellyer Vocab. Criminal Sl. 16: ben [...] An overcoat derived from Benjamin, in reference to the biblical coat of many colors.
1930		‘Dean Stiff’ Milk and Honey Route 199: A vest used to be called a ben.
1949		Monteleone Criminal Sl. (rev. edn).
1960		(con. 1950-1960) R.A. Freeman Dict. Inmate Sl. (Walla Walla, WA) 8: Ben – a vest.