It is unlawful for any person to engage in, or carry on, or to represent and advertise himself as engaged in or carrying on the business of plumbing contractor, or sewer contractor in the City of Portland, or to use the words “plumbing contractor,” “sewer contractor,” or “plumber,” or “plumbing,” or expose a sign containing similar import for such purpose, implying that the advertiser is so engaged, unless such person has obtained a registration to engage in and carry on a business of plumbing contractor or sewer contractor in the City of Portland.