Where an interest in expectancy within the meaning of Part I. of the principal Act in any property has, before the thirtieth day of April nineteen hundred and nine, been bonâ fide sold or mortgaged for full consideration in money or money's worth, then no other duty on that property shall be payable by the purchaser or mortgagee when the interest falls into possession than would have been payable if this Part of this Act had not passed, and in the case .of a mortgage any higher duty payable by the mortgagor shall rank as a charge subsequent to that of the mortgagee.