It shall be conclusively presumed that a weight or measure or weighing or measuring device is intended for use in trade or commerce if it is manufactured, offered or exposed for sale or sold for use in this Commonwealth, or is used therein, unless it shall bear a plain, legible, conspicuous and permanent statement to this effect: “Not legal for trade.” It shall be unlawful to use in trade or commerce any weight or measure or weighing or measuring device which is marked as described above, provided, however, that, whenever it shall appear to the satisfaction of the bureau that any type of weight or measure or weighing or measuring device is such as to render it impracticable to mark it as required by this section or is of such design and construction that it is obviously not intended for use in trade or commerce, the bureau shall furnish a certificate to that effect to any manufacturer applying for the same, and such types of weights and measures and weighing and measuring devices need not be marked as required by the provisions of this section.