If for any reason any person has not been assessed or has been under-assessed for any financial year the assessing authority may, notwithstanding anything contained in sub-section (2) of section 7, at any time within five years of the end of that year, serve on the person liable to pay the tax, a notice containing all or any of the requirement, which may be included in the notice under sub-section (2) of section 7 and may proceed to asses or reassess him under the provisions of this Act shall, so far as may be, apply accordingly as if the notice were a notice issued under that sub-section:

Provided that the tax shall be charged at the rate at which it would have been charged if the person would not escaped assessment or full assessment, as the case may be.