Green’s Dictionary of Slang

(UK Und.) half-a-crown, 2s 6d (12½p).

c.1698		B.E. Dict. Canting Crew n.p.: Slate c. a half Crown.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].