Mont. Code Ann. § 33-15-302

The policy, when issued, shall contain the entire contract between the parties, and neither the insurer or any insurance producer or representative thereof nor any person insured thereunder shall make any agreement as to the insurance which is not plainly expressed in the policy. This provision shall not be deemed to prohibit the modification of a policy, after issuance, by written rider or endorsement duly issued by the insurer.