Green’s Dictionary of Slang

(Scot./Aus. prison) a helper, an assistant.

1962		B. McGhee Cut and Run (1963) 58: Dae ye think Ah’ve nae handers? Ah can get ye sortit, nae bother.
1973		(con. mid-1960s) J. Patrick Glasgow Gang Observed 65: Twenty members of the ‘Toon’ had arrived [...] With only two ‘handers’ he had prevented them from gaining entrance.
1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Hander. A person who provides assistance to someone in trouble.