Federal Reserve notes shall bear upon their faces a distinctive letter and serial number which shall be assigned by the Board of Governors of the Federal Reserve System to each Federal Reserve bank. Federal Reserve notes unfit for circulation shall be canceled, destroyed, and accounted for under procedures prescribed and at locations designated by the Secretary of the Treasury. Upon destruction of such notes, credit with respect thereto shall be apportioned among the twelve Federal Reserve banks as determined by the Board of Governors of the Federal Reserve System.