Section 56C. Every municipal light commission or manager thereof, who makes or executes a contract on behalf of a municipal lighting plant, where the amount involved is five thousand dollars or more, shall furnish said contract or a copy thereof to the city or town auditor within one week after its execution. Said city or town auditor shall keep such contract or copy on file, open to public inspection during business hours. Such contracts or copies shall be kept in a separate book, arranged according to the subject of the contract, or in other convenient form. An index of the subject matter of the contracts and to the names of the contractors shall be made semi-annually, and shall also be open to public inspection in some convenient form. All allowances under and additions to such contracts, or copies thereof, shall be filed with the city or town auditor, together with a sworn statement of the officer making such allowances or additions that the same are correct and in accordance with the contract. A city or town auditor, municipal light commissioner or manager wilfully failing to comply with this section shall be punished by a fine of not less than ten nor more than one hundred dollars.