Green’s Dictionary of Slang

(UK Und.) an oxy-acetylene blowlamp, as used by safe-crackers.

1931		E. Blair ‘Hop-Picking Diary’ 19 Sept.–8 Oct. in Complete Works X (1998) 231: Bly, a ... an oxy-acetylene blowlamp.
1933		‘George Orwell’ Down and Out in Complete Works I (1986) 176: These (omitting the ones that everyone knows) are some of the cant words now used in London: [...] A bly – a burglar’s oxy-acetylene blow-lamp.