law: 24. Turnover escaping assessment.
  (1) Where after a dealer has been assessed under section 23 for any year or part thereof, the Commissioner has reason to believe that the whole or any part of the turnover of a dealer in respect of any period has escaped assessment to tax or has been under-assessedor has been assessed at a rate lower than the rate at which it is assessable, or any deduction has been wrongly made therefrom, the Commissioner may
  (a) within six years from the date of final order of assessment, in a case where the dealer has concealed, omitted or failed to disclose fully the particulars of such turnover; and
  (b) within four years from the date of final order of assessment, in any other case,
  serve a notice on the dealer and after giving the dealer an opportunity of being heard and making such inquiry as he considers necessary, proceed to determine to the best of his judgment, the amount of tax due from the dealer in respect of such turnover, and the provisions of this Act shall, so far as may be, apply accordingly.
  Explanation.
  For the purposes of this section, production before the Commissioner of account books or other evidence from which material evidence could with due diligence have been discovered by the Commissioner will not necessarily amount to disclosure within the meaning of this section.
  (2) No order of assessment, reassessment or re-computation shall be made under sub-section (1), after
  (a) the expiry of four years or, as the case may be, six years as specified in sub-section (7) of section 23; or
  (b) the expiry of one year from the date of service of notice under sub-section(1), whichever is later.