Rules and executive instructions issued before commencement of this Act, to be followed for the purpose of assessment operations begun before issue of rules made under the provisions of section 65.

Notwithstanding anything contained in section 65 for the purpose of all assessment operations begun before the date of publication of rules made after the commencement of this Act, the rules and executive instructions relating to the matters mentioned in clauses (a), (b), (c) and (d) of section 64 which were in force before such publication shall remain in force.