The Commissioner may either of his own moti on or on application made by an assessee in this behalf, ca ll for the record of any proceeding under this Act in which an order has been passed by any authority subordinate to hi m, and may make such inquiry, or cause such inquiry to be mad e, and, subject to the provisions of this Act, pass such ord er thereon, not being an order prejudicial to the assessee, as the Commissioner thinks fit; Provided that the Commissioner shall not revise a ny order under this sub section in any case (a) where an appeal against the order lies to t he Appellate Assistant Commissioner or to the Appellate Trib u nal, the time within which such appeal can be made has n ot expired or in the case of an appeal to the Appellate Trib u nal the assessee has not waived his right of appeal; (b) where the order is the subject of an appe al before the Appellate Assistant Commissioner or the Appella te Tribunal.