Section 16. Every person engaged in the business of a master plumber or a master gas fitter or working as a journeyman plumber or a journeyman gas fitter or as an apprentice gas fitter or an apprentice plumber or as an undiluted liquefied petroleum gas installer or a limited undiluted liquefied petroleum gas installer not lawfully registered or licensed, if required by the chapter, every person engaging in or working at the business of plumbing or gas fitting in a city or town when forbidden to do so under section seven, every master plumber or master gas fitter who engages or employs any person to work as a journeyman or as an apprentice who has not been so licensed, and every person so licensed violating any provision of sections one to fifteen, inclusive, of this chapter or any ordinance, by-law, rule or regulation made thereunder, shall be punished by a fine not exceeding one hundred dollars. Any city or town subject to the preceding sections of this chapter neglecting to comply with any of its provisions shall forfeit fifty dollars to the commonwealth for each month during which such neglect continues.