S. 263: Commissioner-Revision of orders prejudicial to revenue-Capital introduced by partners-Burden to explain source lies on partners-Assessment of firm not erroneous-Cash deposits in bank-Cash sales recorded in books and accepted-Revision invalid-Audit objection-Internal audit information can form basis-Independent application of mind by Principal Commissioner sufficient-Unsecured loan-Issue not challenged by assessee-Matter restored to Assessing Officer.[S.68, 143(3)]