Service of any notice or other document under the Act or these Regulations may be effected: ¾

(1) in the case of an industrial union, in accordance with section 60 (3) of the Act;

(2) in the case of a company or other corporation, by leaving it at, or sending it by post in a prepaid registered letter to the registered office of the company or corporation;

(3) In the case of a partnership, firm or unincorporated company or body, by leaving it at, or sending it by post in a prepaid letter to the partnership, firm or unincorporated company or body at the principal place of business thereof in the State;

(4) In the case of any other person by delivering it to him personally, or by leaving it for him at his usual or last known place of abode, or if he is in business, at his usual or last known place of business, or sending it by post in a prepaid letter addressed to him at his usual or last known place of abode, or if he is in business, at his usual or last known place of business: Provided that no place shall be deemed the place of business of any person unless he is a principal in the business.