In the case of any person dying on or after the thirtieth day of April nineteen hundred and nine, section twenty of the [59 & 60 Vict. c. 28.] Finance Act, 1896 (which gives an exemption for objects of national, scientific, or historic interest), shall be extended so as to give an exemption from legacy and succession duty as well as from estate duty, and as so extended shall take effect whether the property in respect of which the exemption is given is settled or not, and as if the reference therein to national, scientific, or historic interest included a reference to artistic interest, and duty shall only become chargeable when the property is sold, and then only in respect of the last death on which the property passed.