When an instrument is marked by the Chief Commissioner or Commissioner of State Revenue as duly stamped, as fully stamped, or as sufficiently stamped, a purchaser or the Registrar-General or the Crown Solicitor shall not by reason only of the stamp borne by the instrument, or of its being so marked as aforesaid, be deemed to have notice of any trust or of any written contract of sale or other document affecting the title of the property to which the instrument refers; but notwithstanding anything in this section the Registrar-General may, if the Registrar-General thinks fit, require production of any such document.