Obligation to make additional declaration and to pay further tax

Whenever any person, who has delivered a declaration under section 4 and 4-A becomes liable to an additional tax by reason of his keeping a greater number of motor vehicles for use than he has stated in the declaration or by reason of any change in the character of any motor vehicle kept by him for use, he shall fill up and sign an additional declaration specifying with reference to such liability the particulars required under section 4 and 4-A. Such person shall deliver the additional declaration so filled up and signed and pay such additional tax as by the last mentioned declaration appears to be payable by him to the licensing officer before the expiration of 21 days from the day of his becoming so liable as aforesaid: Provided that when payment is made of additional tax by reason of any change in the character of any motor vehicle, an allowance shall be made for the tax already paid.