Some employees are covered because their work, although not actually a part of such production, is “closely related” and “directly essential” to it. This group of employees includes bookkeepers, stenographers, clerks, accountants and auditors and other office and white collar workers, and employees doing payroll, timekeeping and time study work for the producer of goods; employees in the personnel, labor relations, advertising, promotion, and public relations activities of the producing enterprise; work instructors for the producer; employees maintaining, servicing, repairing or improving the buildings, machinery, equipment, vehicles or other facilities used in the production of goods for commerce, and such custodial and protective employees as watchmen, guards, firemen, patrolmen, caretakers, stockroom workers, and warehousemen; and transportation workers bringing supplies, materials, or equipment to the producer's premises, removing waste materials therefrom, or transporting materials or other goods, or performing such other transportation activities, as the needs of production may require. These examples are illustrative, rather than exhaustive, of the group of employees of a producer who are “engaged in the production of goods for commerce” by reason of performing activities closely related and directly essential to such production.