Mont. Code § 15-30-2619
Each taxpayer shall, upon request of the department, furnish a copy of the return for the corresponding year that the taxpayer has filed or may file with the federal government, showing the taxpayer's net income and how obtained and the several sources from which derived. Except as provided for in 15-30-3403 or 15-30-3404, if the amount of a taxpayer's taxable income is changed or corrected by the United States internal revenue service or other competent authority, the taxpayer shall file an amended Montana return with the department within 180 days after the final determination date. Except as provided for in 15-30-3403 or 15-30-3404, if a taxpayer files an amended federal income tax return changing or correcting the taxpayer's federal taxable income for a tax year, the taxpayer shall also file an amended Montana return with the department within 180 days after filing an amended federal income tax return. The department shall supply all necessary forms and shall, upon the request of the taxpayer, return all forms to the taxpayer after they have been examined by the department.
§ 15-30-2619, MCA