Green’s Dictionary of Slang

holler boys, holler n.

a (stiff) collar.

1930		(con. 1914–18) Brophy & Partridge Songs and Sl. of the British Soldier.
1943		R. Llewellyn None But the Lonely Heart 114: ‘Where’s your holler boy’s, holler?’ [...] ‘I catch on,’ he says. ‘I just come from work. I don’t wear one.’.
1960		J. Franklyn Dict. of Rhy. Sl.
1969		S.T. Kendall Up the Frog 12: This ’ere bloke grabs ’im by the’oller boys ’oller and gives ’im a real fourpenny one in the Newington Butts.
1972		Dodson & Saczek Dict. of Cockney Rhy. Sl.
1980		Barltrop & Wolveridge Muvver Tongue 19: Collar : a holler boys holler.
1984		Partridge DSUE (8th edn) 561/2: holla (or holler or holloa) boys holla (etc.) often shortened to holla (etc.) boys A collar.