13.—(1) Where an instrument is chargeable with ad valorem duty in respect of —
(a)
any money expressed in any currency other than that of Singapore; or
(b)
any stock or marketable or other security,
the duty is to be calculated on the value, on the day of the date of the instrument, of the money in the currency of Singapore according to the current rate of exchange, or of the stock or security according to the average price of the stock or security or, if there is no price, according to the value of the stock or security.
(2) Where an instrument contains a statement of current rate of exchange or average price (as the case may require) and is stamped in accordance with that statement, the instrument, so far as regards the subject matter of the statement, is presumed, until the contrary is proved, to be duly stamped.