If, at the time of his appointment to the Supreme Court, a Judge is in receipt of a pension in respect of any previous service either as a Judge of a High Court or in any other pensionable civil post under the Union or a State, the pension payable to him under this Act shall be an additional pension for service in the Supreme Court equal to the difference between his original pension and the pension to which he would have been entitled under this Act, if his service in the Supreme Court had been rendered in continuation of the previous service for which his original pension was granted.