Section 50A. Every person employed as a watchman in establishments other than those described in section forty-eight, or employed in maintaining fires in such establishments, but not including janitors in residential apartment houses, shall be allowed at least twenty-four consecutive hours of rest in every seven consecutive days. No provision of any other section of this chapter shall be construed as limiting the rights given by this section. The term ''watchman'' as used in section forty-eight or the term ''watchman'' as used in this section shall include guards in banks, as defined in section one of chapter one hundred and sixty-seven. An employer violating this section shall be punished by a fine of three hundred dollars.