No policies of insurance on life issued prior to the operative date of section 376.670 by any life insurance company authorized to do business in this state shall, after payment upon it of three or more annual payments, be forfeited or become void by reason of nonpayment of premiums thereon, but it shall be subject to the following rules of commutation, to wit: The net value of the policy, when the premium becomes due and is not paid, shall be computed upon the actuaries’ or combined experience table of mortality with four percent interest per annum, and after deducting from three-fourths of such net value the unpaid portion of any notes given on account of past premium payments on said policy and any other indebtedness to the company secured by said policy, which notes and indebtedness shall then be cancelled, the balance shall be taken as a net single premium for temporary insurance (extended insurance). The amount of such temporary insurance shall be such as is specified in the policy, but never less than the face amount insured by the policy reduced by the unpaid portion of notes and indebtedness aforesaid; and the term for which said temporary insurance shall be in force shall be determined by the attained age of the person whose life is insured at the time of default of premium, the assumption of mortality and interest aforesaid, the amount of temporary insurance granted and the net single premium available for temporary insurance previously defined; provided, that if the original policy shall be an endowment, payable at a certain time or at death, if it should occur previously, then if the net single premium available for temporary insurance as aforesaid shall exceed the net single premium for temporary insurance granted for the remainder of the endowment term, such excess shall be considered as a net single premium for pure endowment of as much as said excess of premiums will purchase, determined by the attained age of the insured at date of default in the payment of premiums on the original policy, and the table of mortality and interest aforesaid, which amount of pure endowment shall be paid at the end of the original term of endowment if the insured shall then be alive.