Section 46B. Notwithstanding the other provisions of this chapter no person shall open, keep, maintain, carry on, or advertise any employment agency unless he has been issued a license therefor under section forty-six D from the commissioner; provided, however, that a license validly issued pursuant to section one hundred and eighty C shall suffice for the purposes of this section. Such license shall be posted in a conspicuous place in each employment agency conducted by such person. Licensing of theatrical booking agents, personal agents and managers shall be governed by sections one hundred and eighty A to one hundred and eighty G, inclusive. A firm specifically excluded by section forty-six A as an employment agency shall by mail notify the commissioner of its name and address and any change thereof.