“Approved milk inspection service” means a milk inspection unit and laboratory which are maintained in connection with a county for the purpose of grading market milk and inspecting dairy farms which are producing market milk, or in connection with a county or city for the purpose of inspecting establishments as provided for in Section 33704, within an area which is designated and assigned by the director, conformable to this division and the regulations established by the director, which inspection service has been approved in writing by the director.