| (a) | - | the 31a[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 31a[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, |