15. If, for any reason profits of any chargeable accounting period chargeable to excess profits tax have escaped assessment, or have been under assessed, or have been the subject of excessive relief, the Excess Profits Tax Officer may at any time serve on the person liable to such tax a notice containing all or any of the requirements which may be included in a notice under section 13, and may proceed to assess or reassess the amount of such profits liable to excess profits tax and the provisions of this Act shall, so far as may be, apply as if the notice were a notice issued under that section: Provided that unless definite information has come into his possession the Excess Profits Tax Officer shall not initiate proceedings under this section without obtaining the previous approval of the Inspecting Assistant Commissioner.