In the pre-decimal days England had £sd. Why did they use the 'd' to designate pennies?

The currency sign is the pound sign. The £ is written with a single cross-bar—this is the style used on sterling bank notes. The pound sign derives from the black-letter "L", an abbreviation of Librae in Roman £sd units (librae, solidi, denarii) used for pounds, shillings and pence in the British pre-decimal duodecimal currency system. Libra was the basic Roman unit of weight, derived from the Latin word for scales or balance.

£sd (sometimes pronounced, and occasionally written, L.s.d.) was the popular name for the pre-decimal currencies (sterling) used in the Kingdom of England, later the United Kingdom and ultimately in much of the British Empire. This abbreviation meant "pounds, shillings, and pence", and was usually pronounced that way, having originated from the Latin words "librae, solidi, denarii". Under this system, there were 12 pence in a shilling and 20 shillings, or 240 pence, in a pound. A penny was, until 1960, further subdivided into four farthings.