Section 215.24 - Books and accounts(a) The lessee shall maintain books in which shall be kept a correct account of all ore and rock mined on the tract, of all ore put through the mill, of all lead and zinc concentrates produced, and of all ore and concentrates sold and to whom sold, the weight, assay value, moisture content, base price, dates, penalties, and price received, and the percentage of lead and zinc recovered. A correct statement of the same for each month shall be furnished the office of the district mining supervisor pursuant to § 215.23 not later than 15 days after the first of each month for the preceding month, together with a certificate from the smelter showing the unit price paid for the mineral purchased and the amount of ore and concentrates purchased during the month from said land.(b) An audit of the lessee's accounts and books shall be made semiannually, or at such other times as may be directed by the Secretary of the Interior, by certified public accountants, approved by the Secretary, and at the expense of the lessee. The lessee shall furnish free of cost a copy of such semiannual or other audit, through the office of the district mining supervisor pursuant to § 215.23 , within 30 days after the completion of each auditing.