Mont. Code Ann. § 33-15-413
Renewal by certificate
33-15-413 . Renewal by certificate. Any insurance policy terminating by its terms at a specified expiration date and not otherwise renewable may be renewed or extended at the option of the insurer and upon a currently authorized policy form and at the premium rate then required therefor, for a specific additional period or periods by certificate or by endorsement of the policy, and without requiring the issuance of a new policy.