Annual reports of compensation paid by insurers, self-insurers and the State Workmen's Insurance Fund shall be made on a calendar year basis to the department not later than April 15 of the following year, except that for the year 1974 reports shall be filed within sixty days of the effective date of this amending act. Nothing in this act shall be construed to preclude insurers from filing its annual report required herein in substantially the same form as its annual report to the Insurance Department.