The tax under section 3 shall be payable by every person specified in that section and falling under one or the other of the classes mentioned in the second column of the Schedule annexed to this Act at the rate mentioned against the class of such persons in the third column of the Schedule:

Provided that entry 21 in the Schedule shall apply only to such classes of persons as may be specified by the State Government by notification from time to time:

Provided further that the State Government may, by notification in the Official Gazette, add to, delete, amend or otherwise modify the said Schedule and also may vary the rates of tax of the entries specified in the Schedule and thereupon the said Schedule shall be deemed to have been amended accordingly.