(b)notwithstanding that there has been no omission or failure as mentioned in clause (a) on the part of the assessee, the [Assessing Officer] [Substituted by Act 26 of 1988, section 72, for 'Income-tax Officer' (w.e.f. 1-4-1988).] 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,
