Green’s Dictionary of Slang

(Aus. und.) a skeleton key.

1895		Crowe Aus. Sl. Dict. 89: Turns, skeleton keys.
1901		Argus (Melbourne) 20 Sept. 6/4: He carries, his a rule, his faked turns, or skeleton keys, and his shady glim, or a dark lantern.
1923		Smith’s Wkly (Sydney) 11 Aug. 15/2: Effect entrance either by door or window. In the case of the Rory o’ More, a skeleton turn wlll do the trick.