No general mortgage bond registered after the thirty-first day of December, 1916, shall confer any preference in respect of immovable property, and no general clause in a mortgage bond hypothecating immovable property registered after the said date shall confer any preference in respect of any property: Provided that the preceding provisions of this section shall not affect any preference conferred by a general clause in any mortgage bond passed before the commencement of this Act by a widower or widow in favour of a Master, for the purpose of securing the payment to his or her child of any sum of money due to the child from the estate of the widower's or widow's deceased spouse.