In the case of a person who at a time immediately before 1st April 1974 held office as a stipendiary magistrate under section 29 of the Justices of the M1Peace Act 1949 contributions to his pension under section 7 of this Act, and to any derivative benefit, shall be paid and borne under the law in force at that time, and as if his service as a stipendiary magistrate after the beginning of April 1974 had been service as a metropolitan stipendiary magistrate.