The Purchasing Agent shall have on file in his office, for each equipment purchase of $300.00 or more, a copy of the purchase order denoting the Pierce County property number thereon, with a copy of the approved invoice and any other documentation required for a satisfactory audit. In addition, a copy of this information shall be furnished to the ordering department as historical documentation and audit back-up. These files shall be known as the inventory control file and shall be filed separately from other purchasing records.