Green’s Dictionary of Slang

1. the vagina.

1740		‘Sou Ma Quang’ Chinese Tale 23: The World’s great Primum Mobile; / That Master-piece! that Source of Passion! / That Thing! that’s never out of Fashion.
1890–1904		Farmer & Henley Sl. and Its Analogues.

2. the penis.

1765

Harris’s List of Covent Garden Ladies 45: [H]e slily conveyed into her virgin hand the stately masterpiece of nature.

3. (Aus. und.) a skeleton key as used by sneak-thieves.

1865		Age (Melbourne) 28 June 7/5: Detectives [...] having found a variety of housebreaking tools [...] at his residence. These consisted of picklocks, skeleton keys, files, rasp, vice, and what in slang is termed a ‘masterpiece,’ a piece of wire so formed as to penetrate through the keyhole of a door, and turn the key left in the lock inside.
1900		Geelong Advertiser (Vic.) 3 Feb. 1/4: Henry Teagno, who is regarded by the detective police as an unusually clever member of the hotel barbering fraternity, got 12 months’ imprisonment to-day owing to his having had the misfortune to be in possession of what is known as a masterpiece [...] The masterpiece is an instrument which, when a door is looked from the inside and the key-left in, grips the end of the key protruding outwards and turns it round.