Assessing Officer substitution expands Commissioner's record review powers and includes rehearing time in procedural time computation. Amendment replaces 'Income-tax Officer' with 'Assessing Officer' in section 19(1) and substitutes an Explanation declaring that 'record' includes all records relating to any proceeding under the Act available at the time of examination by the Commissioner, and that the Commissioner's power extends to matters not considered in any appeal. It also amends section 19(3) Explanation to include the time taken to give the assessee an opportunity to be reheard under the Income-tax proviso, for purposes of time computation in the Act.