Four-fifths of the reserve of 15 per centum which a national bank located in a dependency or insular possession or any part of the United States outside of the continental United States, and not a member of the Federal Reserve System, is required to keep, may consist of balances due such bank from associations approved by the Comptroller of the Currency and located in any one of the reserve cities as now or hereafter defined by law or designated by the Board of Governors of the Federal Reserve System.