If--
(a) the [Assessing Officer] has reason to believe that by reason of omission or failure on the part of the assessee to make a return under section 8 for any assessment year or to disclose wholly and truly all material facts necessary for his assessment for any assessment year, chargeable expenditure for that year has escaped assessment or has been under-assessed, or
(b) notwithstanding that there has been no omission or failure as mentioned in clause (a) on the part of the assessee, the [Assessing Officer] has, in consequence of information in his possession, reason to believe that chargeable expenditure assessable in any assessment year has escaped assessment or has been under-assessed,
he may, in cases falling under clause (a), at any time, and in cases falling under clause (b), at any time within four years from the end of that assessment year, serve on the assessee a notice containing all or any of the requirements which may be included in a notice under section 8 and may proceed to assess or reassess the chargeable expenditure, and the provisions of this Act shall, so far as may be, apply, as if the notice was a notice issued under that section.