Mont. Code Ann. § 61-3-728

An owner whose application for proportional registration has been accepted shall preserve the records on which the application is based for a period of 4 years following the year or period upon which the application is based. Upon request of the department, the owner shall make these records available to the department for audit as to accuracy of computations and payments or pay the reasonable costs of an audit at the owner's home office by an appointed representative of the department. The department may make arrangements with agencies of other jurisdictions administering motor vehicle, trailer, semitrailer, or pole trailer registration laws for joint audits of the owner.