Mont. Code Ann. § 61-3-719

If an agreement, arrangement, or declaration is not in effect with respect to another jurisdiction as authorized by 61-3-711 through 61-3-733, any motor vehicle, trailer, semitrailer, or pole trailer properly registered or licensed in another jurisdiction and for which evidence of compliance is supplied must receive, when operated in this state, the same exemptions, benefits, and privileges granted by the other jurisdictions to motor vehicles, trailers, semitrailers, or pole trailers properly registered in this state. Reciprocity extended under this section applies to commercial motor vehicles, trailers, semitrailers, or pole trailers only when engaged exclusively in interstate commerce.