Mont. Code Ann. § 72-3-609

If any property not included in the original inventory comes to the knowledge of a personal representative or if the personal representative learns that the value or description indicated in the original inventory for any item is erroneous or misleading, the personal representative shall make a supplemental inventory or appraisal showing the market value as of the date of the decedent's death of the new item or the revised market value or descriptions and the appraisers or other data relied upon, if any. The personal representative shall file the supplemental inventory with the court if the original inventory was filed or shall furnish copies of the supplemental inventory or information to persons interested in the new information.