Pennyweight
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A pennyweight (symbol dwt) is an obsolescent unit of mass which is the same as 24 grains, 1/240th of a troy pound, 1/20th of a troy ounce, or approximately 1.555 grams. So, a penny coin was literally, as well as monetarily, 1/240th of a troy pound of sterling silver.
The troy pound and the pennyweight were outlawed in the United Kingdom of Great Britain and Ireland in the Weights and Measures Act of 1878. Only the troy ounce and its decimal subdivisions remained legal after then, and even now in the 21st century the troy ounce enjoys a specific exemption from their metrication laws.