(2) Returns. Any return of a United States shareholder required to be filed before the completion of a period with respect to which determinations are to be made as to a controlled foreign corporation's investments in export trade assets for purposes of computing such shareholder's taxable income shall be filed on the basis of an estimate of the amount of such corporation's investments in export trade assets at the close of the period. If the actual amount of such investments is not the same as the amount of the estimate, the shareholder shall immediately notify the Commissioner. The Commissioner will thereupon redetermine the amount of such shareholder's tax for the year or years with respect to which the incorrect amount was taken into account. The amount of tax, if any, due upon such redetermination shall be paid by the shareholder upon notice and demand by the district director. The amount of tax, if any, shown by such redetermination to have been overpaid shall be credited or refunded to the shareholder in accordance with the provisions of sections 6402 and 6511 and the regulations thereunder.