Green’s Dictionary of Slang

crack n.5

[? perversion of SE crown]

a crown.

[UK]C. Day Lewis Otterbury Incident 112: A simple, ordinary coin of the realm, vulgarly known as half a crack or a demi-dollar.

In phrases

half-a-crack (n.)

half-a-crown, 2s 6d (12½p).

in R. Knox Body in the Silo [DSUE].