I am no expert on the Montana Registration but here is what the google research engine says. In essence you are a business in Montana under a LLC business structure.
Here's how it works: You hire a Montana law firm, they set up an LLC for you, they charge you a fee, and then they register your vehicles in Montana. Not only does this save you money, it also saves you time and aggravation, since the Montana business goes to the DMV for you.