18. Liquid capacity measures, if their maximum purported values are clearly defined, may have a top rim, lip or retaining edge to prevent spilling:
(a)in the case of measures made of metal for the sale of milk and in the form of churns, the top rim, lip or retaining edge shall not increase the capacity of the measure by more than 25 per cent. of its maximum purported value;
(b)in the case of other measures, the top rim, lip or retaining edge shall not increase the capacity of the measure by more than 10 per cent. of its maximum purported value.