The proper officer shall, within a period not exceeding six months from the date of the communication of the order issued under sub-section (1), pass the final assessment order after taking into account such information as may be required for finalizing the assessment:
Provided that the period specified in this sub-section may, on sufficient cause being shown and for reasons to be recorded in writing, be extended by the Joint Commissioner or Additional Commissioner for a further period not exceeding six months and by the Commissioner for such further period not exceeding four years.