Section 29. No notice or extension of restriction under sections twenty-seven or twenty-eight shall be effective against a subject parcel (a) if its title is registered, unless the notice or extension is noted on the certificate or certificates of title thereof or (b) if its title is not registered, unless (1) the notice or extension is indexed in the grantor index under the names of the persons named therein as owners of the subject parcel, (2) if the instrument imposing the restriction is recorded at a registry of deeds, the notice or extension is noted on the margin of the record of the instrument, and (3) if the instrument imposing the restriction is a will, a duplicate or certified copy of the notice or extension is filed with the records of the probate of the will. No notice under clause (b) of section twenty-eight shall be effective unless indexed in a special index which each register shall maintain arranged alphabetically by city or town and within each city or town, by the ways named, in which are listed the books and pages of record of the notices of restriction and of the instruments therein specified as creating them. Where an instrument imposes more than one restriction a notice or extension may, if it so specifies, apply only to a particular restriction or restrictions. A notice under section twenty-seven or twenty-eight may be given with respect to any number of parcels subject to the restriction and may be joined in by the owners of any number of parcels having the benefit thereof and may be signed in behalf of any person by an attorney or agent and on behalf of any person under disability by a guardian, conservator or parent. No notice given under section twenty-seven or twenty-eight shall entitle any person to enforce a restriction other than the person giving the notice and his successors in title, nor entitle anyone to enforce a restriction if at the time of recording of the notice the restriction is for any reason no longer enforceable.