Section 40. A sealer or deputy visiting the place of business of a person to test and seal weighing or measuring devices may use therefor suitable weights, measures and balances; and each town and district shall furnish its sealer with one or more duplicate sets of weights, measures and balances, which shall at all times be kept to conform to the standards provided by the commonwealth; and all weighing or measuring devices so sealed shall be deemed legally sealed, as if tested and sealed with the standards.