Every stock company, mutual carrier and reciprocal exchange shall, on or before the first day of September, one thousand nine hundred and thirty-eight, file with the Treasurer of the Commonwealth, and with the commissioner, identical returns under oath, on a form to be prescribed and furnished by the commissioner, stating the amount of net written premiums for the year's period ending June thirtieth, one thousand nine hundred and thirty-eight, on policies issued, renewed, or extended by such stock company, mutual carrier and reciprocal exchange to insure payment of compensation pursuant to the Workmen's Compensation Law. For the purposes of this act, “net written premiums” shall mean gross written premiums, less return premiums on policies returned not taken and on policies cancelled. Thereafter on or before the first day of September of each year, each such stock company, mutual carrier and reciprocal exchange shall file similar identical returns, stating the amount of such net written premiums for the year's period ending on the preceding June thirtieth on policies issued, renewed, or extended by such stock company, mutual carrier and reciprocal exchange.