The board may apply to reduce the book value of securities purchased, all or part of the excess of the proceeds of the sale or redemption prior to maturity of securities over the book value of the securities sold or redeemed provided the purchase of securities is made with those proceeds and provided that the terms of both securities from the date of sale, redemption, or purchase, as the case may be, to the respective dates of maturity, do not differ by more than five years. All applications of excess of sales or redemption proceeds, even with greater difference in terms, made by the board before October 1, 1949, are hereby validated and confirmed.