Section 9. Every member of, or subscriber to, a rating organization shall adhere to the filings made on its behalf by such organization except that any such insurer may make written application to the commissioner for permission to file a uniform percentage decrease or increase to be applied to the premiums produced by the rating system so filed for a kind of insurance, or for a class of insurance which is found by the commissioner to be a proper rating unit for the application of such uniform percentage decrease or increase, or for a subdivision of a kind of insurance (1) comprised of a group of manual classifications which is treated as a separate unit for rate making purposes, or (2) for which separate expense provisions are included in the filings of the rating organization. Such application shall specify the basis for the modification and shall be accompanied by the data upon which the applicant relies. A copy of the application and data shall be sent simultaneously to such rating organization. The commissioner shall set a time and place for a hearing at which the insurer and such rating organization may be heard and shall give them not less than ten days' written notice thereof. In the event the commissioner is advised by the rating organization that it does not desire a hearing he may, upon the consent of the applicant, waive such hearing. The commissioner shall issue an order permitting the modification for such insurer to be filed if he finds it to be justified and it shall thereupon become effective. He shall issue an order denying such application if he finds that the modification is not justified or that the resulting premiums would be excessive, inadequate or unfairly discriminatory. Each deviation permitted to be filed shall be effective for a period of one year from the date of such permission unless terminated sooner with the approval of the commissioner.