Citations:metric ounce

31¼ grams

 * 1869 July, Decimal-Point (pseudonym), “The French and English Systems of Money-Weights and Measures” in The Pharmaceutical Journal and Transactions, Second Series, volume XI, № i, page 32:
 * 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.

25 grams or CC

 * see the citations in the entry

28.25 grams

 * 2013, Lianne C Webster, Pharmacy Practice for Today's Pharmacy Technician ISBN 0323085792, page 190:
 * The metric ounce weight measurement is 28.35 g (Table 10-2)—not 28 g, not 29 g.

30 grams

 * see the citations in the entry

???

 * 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.

what other dictionaries have to say

 * 2008, The Dictionary of Unfamiliar Words ISBN 1602393397:
 * metric ounce  A unit of mass equal to 25 grams. Also called a mounce.