In our coppers we have fifths and tenths both of the pound and the ounce, so that a farthing thrown into the scale with our ounce is the equivalent (within one grain) to the so-called metric-ounce or sixteenth of 500 grammes, and with eight halfpence on the pound we get the half-kilo quite near enough for practical purposes.
1870, French and English Systems of Money-Weights and Measures, in The Pharmaceutical Journal , page 31:
The practice in the metric system is to treat the metric atom or the gramme as a weight equal to 4 gros; the so-called metric ounce of 31 1/4 grammes is made one of 30, so that 30 in the metric system is taken for 31 1/4. […] in practice they are compelled to accept a binary scale of 30 grammes as an ounce.