Green’s Dictionary of Slang

a piece of celluloid used for picking Yale locks; also as v., to pick a lock with a piece of celluloid.

1959		J. Gosling Ghost Squad 24: Thieves’ argot, spoken properly, is a foreign language which needs to be learned [...] Among the words and phrases derived from rhyming slang are: [...] Harold (Harold Lloyd) is celluloid, an instrument of housebreaking.
1977		D. Powis Signs of Crime 191: Lloyd (Harold) See Loid; Loid (Harold Lloyd) Celluloid (or, nowadays, plastic pieces) used to slip door catches when housebreaking: ‘When they nicked him he had loid in his pocket.’.
1989		in G. Tremlett Little Legs 37: I could lloyd the drawer, using a piece of plastic like a Barclaycard — a piece of lloyd (celluloid).
1989		J. Morton Lowspeak 71: Harold – celluloid.