Section 6. If the amount of tax imposed by Title III of said revenue act or any subsequent federal revenue act shall be increased or decreased as affecting an estate taxable hereunder subsequent to the payment of the tax on account of such estate imposed by section one of this chapter the tax imposed upon such estate hereunder shall be changed accordingly. Any additional tax shall be assessed by the commissioner and paid within thirty days after the date of the notice from the commissioner of the amount thereof. Any excess tax received by the commonwealth shall be refunded within thirty days after the amount shall have been certified by the commissioner without appropriation; provided that no refund shall be made under this section unless written application therefor is made to the commissioner within six months of the date of final determination of the federal tax.