Every person engaged in those businesses and trades covered by this article shall
display on all vehicles used to transport materials and tools in the operation of
the business, the firm name and the license number bearing the initials "OK" preceding
that number issued to the electrical contractor by the state. Such signs and license
numbers shall be printed in letters and numerals at least two inches high in conspicuous
places on both sides of the vehicle in contrasting color to background surface.
