Mont. Code Ann. § 70-1-308

When so specified in the agreement granting for a term of time the right in two or more persons to use or occupy any safe or box, commonly referred to as a safe deposit vault or box for the safekeeping of valuables, such interest and estate created in the grantees shall be a joint tenancy in such vault or box and pass to the survivors and survivor upon the death of one or more of the joint tenants with right in such survivors and survivor to have access to and possession of such vault or box and the contents thereof under the terms of the agreement.