Mont. Code Ann. § 61-6-133

Proof of financial responsibility may be furnished by filing with the department the written certificate of any insurance carrier duly authorized to do business in this state certifying that there is in effect a motor vehicle liability policy for the benefit of the person required to furnish proof of financial responsibility. Such certificate shall give the effective date of such motor vehicle liability policy, which date shall be the same as the effective date of the certificate, and shall designate by explicit description or by appropriate reference all motor vehicles covered thereby, unless the policy is issued to a person who is not the owner of a motor vehicle.