S. 154 : Rectification of mistake-Mistake apparent from the record -Property held for charitable purposes-Failure to file Form No 10 along with return of income-Form was filed before the Assessing authority before completion of assessment-Eligible for exemption-Rejection of rectification application was not valid. [S. 11(2)(c), 143(1), 154, R. 17, Form No. 10]