A half crown piece is a fore coach wheel, and a crown piece a hind coach wheel; the fore wheels of a coach being less than the hind ones.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1736 dictionary of canting and thieving slang.
Clyster Pipe * To CoaxNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes