Annual reports shall be filed with the department on or before the fifteenth day of March. The department may grant a reasonable extension of time, not to exceed sixty days, upon written application of the company filed with the department on or before the fifteenth day of March, and showing good cause why such an extension is required. In the event any other report required to be filed with the department, e.g., annual stockholders report or regulatory agency report, is not available at the time the annual report is filed, the company shall so notify the department and thereafter file such report as soon as it becomes available.