S. 147 : Reassessment-After the expiry of four years-Full and true disclosure-Change of opinion-Books seized not returned-Unable to draw balance-sheet-Subsequently drawing the balance sheet-Assessment under section 143(3) of the Act-Return not treated as defective-Subsequent subjective analysis of Assessing Officer that income of assessee was much higher than that assessed- Change of opinion- Reassessments are not justified-Once primary facts disclosed by assessee, burden shifts to Assessing Officer-Defective return-Burden on Assessing Officer to intimate defect to Assessee for rectification within time specified- Assessing Officer not exercising discretion- Return cannot be construed as defective burden on Assessing Officer to intimate defect to Assessee for rectification within time specified. [S. 139(9)(f), 145(1) 147, 149(1)(b)(iii)]