Mont. Code Ann. § 15-31-326

The department may require a taxpayer making a water's-edge election to submit within 6 months after the taxpayer files its federal income tax return a domestic disclosure spreadsheet to provide full disclosure of the income reported to each state for the year, the tax liability for each state, the method used for allocating or apportioning income to the states, and the identity of the water's-edge corporate group and those of its United States affiliated corporations.