Where tax has been levied at source in excess of the amount of tax chargeable under the provisions of Articles 5 and 7, 10, 11 or 12, applications for the refund of the excess amount of tax have to be lodged with the competent authority of the State having levied the tax, within a period of three years after the expiration of the calendar year in which the tax has been levied. The refund shall be given within a six month period from the on which the application was submitted to the competent authority. The six month period may be extended if the Contracting States agree that the necessary documentation has not been presented to the competent authority of the first-mentioned State.