(1) Where the return submitted under sub-section (1) of section 20 is in the prescribed manner and accompanied by the prescribed documents, the assessment relating to the return period shall, subject to the provisions of sections 22, 24 and section 25, be deemed to have been completed on the receipt of such return.

(2) Where the dealer detects any omission or mistake in the monthly return submitted under sub-section (1), he shall file a revised return rectifying the mistake or omission within two months from the last day of the return period to which the return relates. As a result of such revised return, if the tax payable by the dealer increases, the dealer shall furnish along with such revised return, proof of payment of tax, interest due thereon at the rates specified in section 31 and penal interest calculated at twice the said rate. Subject to the provisions of sections 22, 24 and 25, the assessment relating to the return period shall be deemed to have been completed on the receipt of such revised return.