The justice appointed to fill a vacancy is the successor of the justice whose office became vacant before the expiration of a full term. When a full term expires, the person elected to take the numbered office, as provided in 3-10-201, from that time is the successor.
Who Is The Successor
Mont. Code Ann. § 3-10-513, under Records in Justices' Courts.
Mont. Code Ann. § 3-10-513