Section 81. Mutual fire companies, except as provided in the following section, shall charge and collect upon their policies a full mutual premium in cash or, except as provided in section seventy-three, in notes absolutely payable; provided, that this section shall not be construed to prohibit extension of credit to policyholders with respect to such premium. Any such company shall in its by-laws and policies fix the contingent mutual liability of its members for the payment of losses and expenses not provided for by its cash funds, which liability shall not be less than an amount equal to and in addition to the cash premium written in his policy. The total amount of the liability of the policyholder shall be plainly and legibly stated upon the filing-back of each policy. Whenever any reduction is made in the contingent liability of members, such reduction shall apply proportionally to all policies in force.