Green’s Dictionary of Slang

mother of pearl n.

a girl.

1949		Western Dly Press 12 Jan. 2/3: Other popular rhyming slang terms are [...] ‘mother of pearl’ (girl).
1960		J. Franklyn Dict. of Rhy. Sl.
1972		Dodson & Saczek Dict. of Cockney Rhy. Sl.
1984		Partridge DSUE (8th edn) 757: since ca. 1870 [...] Cockney term occurs only in ‘my old mother of pearl’.
2002		D. Shaw ‘Dead Beard’ at www.asstr.org 🌐 Anyway, I’m sitting there with a glass of bum and stroke, having a tumble down the sink, when a mother of pearl on her pat malone comes alongside.