On purchases of lumber and timber products in quantities of one or more carloads county engineers shall furnish all bidders with identical proposals and specifications, and successful bidder shall be required to deliver materials in all respects the same as were described in the engineer’s specifications, and shall furnish as evidence thereof certificate of inspection by an authorized lumber inspector representing the lumber manufacturers’ associations or any other accredited lumber inspection agency; and also, in the case of materials treated with a preservative an affidavit shall be furnished, signed by an agent of the treating plant, or an agent of an accredited treating inspection agency, setting forth the kind and quality of preservative used, the method of impregnating the wood, and the quantity of the preservative finally retained therein.