(a) any such instrument, and any notarial copy, memorial or extract thereof, may be registered, enrolled or filed if such instrument or such copy, as the case may be, is stamped under section 19 with the amount of duty chargeable on such instrument or on the original of such copy, memorial or extract, as the case may be, under the law in force when the said instrument or original was first executed, and the penalty, if any, prescribed by the said section 19 on stamping the said instrument, or copy, memorial or extract, has been paid; and