The group life insurance policy shall contain a provision that the validity of the policy shall not be contested, except for nonpayment of premium, after it has been in force for 2 years from its date of issue; and that no statement made by any person insured under the policy relating to the insurability of the person shall be used in contesting the validity of the insurance with respect to which such statement was made after such insurance has been in force prior to the contest for a period of 2 years during such person’s lifetime nor unless it is contained in a written instrument signed by the person.