In all cases where superiors have agreed or contracted prior to the passing of this Act to feu land, but have not granted the feu rights thereto prior to the commencement of this Act, and but for the provisions of this Act would have been entitled in the feu rights to stipulate for the payment of casualties as well as of feu-duties, nothing in this Act contained shall be construed or held as depriving such superiors of their right to such casualties, but such casualties may, if desired by such superiors or their vassals in such feus, be converted into annual sums equal to four per cent. on the amount of the price of the redemption of such casualties ascertained as provided in section fifteen hereof, and such annual sums shall be deemed to be feu-duties, with all the legal qualities thereof, and shall form additions to the feu-duties which may have been contracted for, and in the feu rights to be granted of such feus superiors shall be entitled, in the event of the casualties being converted as aforesaid, to state as one feu-duty the feu-duty which has been contracted for or agreed upon, and that formed by the converted casualty, and shall have all competent remedies for recovery thereof. But in the event of neither the superiors nor the vassals in such feus desiring to convert casualties agreed or contracted for before the passing of this Act as aforesaid, it shall be competent in such feu rights to stipulate for payment of the casualties which the vassals may expressly or by force of law have agreed or contracted to pay, in the same manner as might have been done prior to the commencement of this Act; and it shall also be competent in such feu rights to stipulate that the same shall be recorded by the agent of the superior at the expense of the vassal if such or a similar stipulation shall have been made and agreed prior to the passing of this Act in the agreement or contract for feuins: such land.