Such companies shall also make annual returns to the Commissioner of Taxes in form satisfactory to him or her, and shall pay to the Commissioner of Taxes annually, in the month of February, a tax at the rate of two percent on gross premium deposits upon policies on risks located in this State in force on December 31 next preceding, after deducting the unabsorbed portion of such premium deposits computed at the average rate of return actually made on annual policies expiring during such year. However, none of the requirements of any resident agent law of this State shall apply to such a company so long as it insures only factories or mills, or property connected with such factories or mills.