S. 263 : Commissioner-Revision of orders prejudicial to revenue-Assessing Officer calling for further details, making inquiry, examining issue-No discussion in the assessment order-Revision is not valid-Interpretation of taxing statutes-Change of law-Provision inserted from date in midst of financial year, falling within realm of procedural aspects of assessment is to be construed as prospective qua proceedings and not qua assessment year. [S.143(3)]