Mont. Code Ann. § 72-6-307

On the death of a sole owner or the last to die of all multiple owners, ownership of securities registered in beneficiary form passes to the beneficiary or beneficiaries who survive all owners. On proof of the death of all owners and compliance with any applicable requirements of the registering entity, a security registered in beneficiary form may be reregistered in the name of the beneficiary or beneficiaries who survived the death of all owners. Until division of the security after the death of all owners, multiple beneficiaries surviving the death of all owners hold their interests as tenants in common. If no beneficiary survives the death of all owners, the security belongs to the estate of the deceased sole owner or the estate of the last to die of all multiple owners.