Section 23

Any notice, summons, writ or other proceedings re-quired to be served on the Board may.- be served by being left with some person apparently employed in the adminis-tration of this Act at the office of the Board or, in the case of notice, by posting it by registered post in an envelope duly stamped and addressed to the Board at its office, and shall be deemed to have been served upon its being left or, in the case of a notice served by so posting it, shall be prima facie deemed to have been served when it would be delivered in the ordinary course of post.