Green’s Dictionary of Slang

biscuits containing currants, Garibaldi biscuits.

1900		Farmer Public School Word-bk 85: Squashed flies, [...] biscuits with currants.
1914		E. Pugh Cockney At Home 216: Two cups o’ sudden death and some o’ your very best dead-fly cake.
1959		I. & P. Opie Lore and Lang. of Schoolchildren (1977) 184: Garibaldi biscuits are ‘squashed flies’ or ‘fly pies’.
2003		McGill Reed Dict. of N.Z. Sl.