Section 7. If the executor makes a written application, accompanied by a copy of the final determination of federal estate tax liability, to the commissioner for determination of the amount of the tax and discharge from personal liability therefor, the commissioner, as soon as possible, and in any event within one year after the making of such application, or, if the application is made before the return is filed, then within one year after the return is filed, shall notify the executor of the amount of the tax. The executor, on payment of the amount of which he is notified, shall be discharged from personal liability for any deficiency in tax thereafter found to be due, and shall be entitled to a receipt or writing showing such discharge.