It shall be the duty of every person, firm, or corporation engaged in the business of plumbing as an active master plumber to display his certificate of license number on all vehicles while engaged in the business of plumbing as a master plumber.  These numbers shall be attached in a legible manner in contrasting colors on both sides of said vehicles with not less than two-inch block numerals.  When a vehicle displaying a license number is no longer used in the master plumber's plumbing business or in the event the master plumber's license expires or is revoked, the master plumber shall either remove the license number from the vehicle or paint over the license number with an opaque paint so that the license number can no longer be seen.