Mont. Code Ann. § 85-1-305

In the discretion of the department, a series of the bonds may be secured by a trust indenture by and between the department and a corporate trustee, which may be any trust company or bank having the powers of a trust company within or outside of the state. Each trust indenture or an executed counterpart must be filed in the office of the secretary of state. The filing of a trust indenture or an executed counterpart in the office of the county clerk of the county in which the property covered by the trust indenture is located is constructive notice of its contents to all persons from the time of the filing, and the recording of the trust indenture or its contents is not necessary.