Green’s Dictionary of Slang

celluloid or a piece of plastic, such as a credit card, used to slip open Yale-style locks when housebreaking.

1956		‘Charles Raven’ Und. Nights 148: Using his loid he would steam into a house through the front door.
1959		J. Gosling Ghost Squad 28: His nickname [i.e. ‘Celluloid Jack’] derived from the fact that he was the first man to use strips of celluloid as a means of forcing Yale locks.
1976		R. Barnes Coronation Cups and Jam Jars 206: Loid – Celluloid.
1989		J. Morton Lowspeak.
1997		E. Little Another Day in Paradise 3: When to force a door with a pry bar and when you could use a celluloid to slip the lock.