Mont. Code Ann. § 30-12-506

For the purposes of parts 1 through 5, proof of the existence of a weight or measure or a weighing or measuring device in or about any building, enclosure, stand, or vehicle in which or from which is shown that buying or selling is commonly carried on is, in the absence of credible evidence to the contrary, presumptive proof of the regular use of that weight or measure or weighing or measuring device for commercial purposes and of the use by the person in charge of the building, enclosure, stand, or vehicle.