The Board of Governors of the Federal Reserve System may, by regulation, limit to a percentage of the assets of a Federal reserve bank the amount of notes, drafts, acceptances, or bills having a maturity in excess of three months, but not exceeding six months, exclusive of days of grace, which may be discounted by such bank, and the amount of notes, drafts, bills, or acceptances having a maturity in excess of six months, but not exceeding nine months, which may be rediscounted by such bank.