Every person recording such statement or account as provided in the preceding section, except those persons contracting or dealing directly with the owner, part owner or lessee of such premises, shall serve a copy of such statement within 30 days after the recording thereof, upon the owner, part owner, lessee, contractor or sub-contractor or his authorized agent, and upon any persons acquiring any interest in the property after the date of the furnishing of the first materials or the date when the first labor was performed, which said interest shall appear of record in the register of deeds’ office for the county in which the lien claim is recorded, or sent to such owner, part owner, lessee, contractor, sub-contractor or other person having any interest in said property, by registered mail addressed to him at his last known address, within 30 days next after the date of recording such statement, a copy of such statement, and the affidavit of the person so serving or mailing such copy as to the time and manner of service, shall be sufficient proof of the service thereof: Provided, That if the name or address of such owner, part owner, lessee, contractor, sub-contractor or other person having any interest in said property be unknown, in lieu of such service it shall be sufficient if a copy of such statement be posted in a conspicuous place on the premises charged with such lien within said 30 day period.