233.18  Copies of bids, contracts, and instructional materials retained.--Specimen copies of all instructional materials, which have been made the bases of contracts under the provisions of this chapter, shall, upon request for the purpose of public inspection, be made available by the publisher to the Department of Education and the superintendent of each school district that adopts such materials from the state list upon request for the purpose of public inspection. All contracts and bonds executed under the provisions of this chapter shall be signed in triplicate. One copy of each contract and an original of each bid, whether accepted or rejected, shall be preserved in the office of the Department of Education for at least 3 years beyond the termination of the contract.