Section 124. Such order shall be in writing, and may be served personally on the owner, occupant or his authorized agent by any person authorized to serve civil process; or a copy of the order may be left at the last and usual place of abode of the owner, occupant or agent, if he is known and within or without the commonwealth; or a copy of the order may be sent to the owner, occupant or agent by registered mail, return receipt requested, if he is known and within the commonwealth. If the order is directed against the owner and if the residence and whereabouts of the owner or his agent are unknown or without the commonwealth, the board may direct the order to be served by posting a copy thereof in a conspicuous place on the premises and by advertising it for at least three out of five consecutive days in one or more newspapers of general circulation within the municipality wherein the building affected is situated.