Mont. Code Ann. § 82-2-112

The period of time prescribed by this part for the performance of any act may not be considered mandatory if the act is performed before the rights of third persons have intervened, and a defect in the posted notice or recorded certificate may not be considered material except as against one who has located the same ground or some portion of the same ground in good faith and without notice. Notice to an agent who makes a location on behalf of another must be considered notice to the agent's principal, and notice to one of several coclaimants must be considered notice to all.