50. The application for relief under section 49 shall be made within the following periods, that is to say,- (1) in the cases mentioned in clause (d) (5), within two months of the date of the instrument: (2) in the case of a stamped paper on which no instrument has been executed by any of the parties thereto, within six months after the stamp has been spoiled: (3) in the case of a stamped paper in which an instrument has been executed by any of the parties thereto, within six months after the date of the instrument, or, if it is not dated within six months after the execution thereof by the person by whom it was first or alone executed: Provided that,-(a) when the spoiled instrument has been sufficient reasons sent out of [Bangladesh, the application may be made within six months after it has been received back in Bangladesh: (b) when, from unavoidable circumstances, any instrument for which another instrument has been substituted cannot be given up to be cancelled within the aforesaid period, the application may be made within six months after the date of execution of the substituted instrument.