If the rent recoverable under a controlled tenancy for any rental period beginning immediately before the commencement of this Act was, by virtue of section 1(4) of the M1Rent Act 1957 and paragraph 15 of Schedule 16 to the M2Rent Act 1968, the same as the rent recoverable for the rental period comprising the commencement of the Act of 1957 then, after the commencement of this Act, that rent shall remain the rent recoverable under that tenancy for any rental period for which it is neither increased nor reduced under Part II of this Act (but without prejudice to paragraph 1 of this Schedule).