(a) if a dealer receives in any year any amount due to price variations, which would have been included in his turnover for any previous year if it had been received by him in that year, he shall within thirty days from the end of the year in which such amount is received, submit a return in the prescribed form to the assessing authority and thereupon the assessing authority shall proceed to assess the tax payable on such amount;

(b) if the assessing authority is satisfied that ay return submitted under clause (a) is correct and complete, it shall assess the dealer on the basis thereof;

(c) if the return submitted by a dealer under clause (a) appears to the assessing authority to be incorrect or incomplete, the assessing authority shall, after making such inquiry as it may consider necessary and after taking into account all relevant materials gathered by it, assess the dealer to the best of its judgement.

(d) if no return is submitted by the dealer under clause (a), the assessing authority may within four years from the expiry of the period within which such return ought to have been submitted proceed to assess the tax payable on the amount referred to in the said clause: