Green’s Dictionary of Slang

(US Und.) the strongbox within a safe.

1928		J. Callahan Man’s Grim Justice 56: It required only two or three minutes to ‘break down the flunk,’ jimmy the strongbox loose from its fastenings [...] I [...] broke down the ‘flunk’ and frisked the pete from top to bottom.
1931		D. Maurer in Lang. Und. (1981) 43: Inside [...] a top keister and a flunk.
1950		Goldin et al. DAUL 72/2: Flunk. An inner steel compartment in a safe, protected by a small, light door or duster.