Mont. Code Ann. § 7-2-2103

No city, town, or village shall become the temporary or permanent county seat of any county organized under the provisions of part 22 or created by an act of the legislature unless such city or town shall have been incorporated in the manner provided by law or unless such village shall have been regularly platted and a plat thereof filed in the office of the county clerk and recorder and there be fifty qualified electors residing within the boundaries of such platted village.