Mont. Code § 17-5-711
In the discretion of the board of examiners, a series of bonds or notes issued under this part may be secured by a trust indenture by and between the board of examiners and a 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 thereof shall be filed in the office of the secretary of state of Montana. The filing of a trust indenture or an executed counterpart thereof 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.
§ 17-5-711, MCA