Mutual fire or mutual employer’s liability insurance companies incorporated under the laws of other states, which insure only factories or mills or property connected with such factories or mills, may be admitted to this state upon complying with the conditions set forth herein, and shall comply with all the requirements hereof, except that, in lieu of all other taxation upon premiums in this state, such companies shall annually pay a tax at the rate of 2 percent on gross premiums in force on risks in this state after deducting the unabsorbed portion of such premium computed at the rate of return actually made on annual policies expiring during the year by said insurance companies.