Section 15. the Commonwealth or the City, As the Case May Be, Shall

determine within six months after the completion of any public work for which land is taken under this act, or within six months after the filing of a final decree on an appeal taken under this act, whichever shall happen later, with which of the adjoining properties the public interests require that each parcel of land, if any, taken outside the boundaries of the public work should be united; and shall, within said six months, notify the owner of such adjoining property, if his address is known, of this decision by registered letter mailed to such owner, and shall annex to the notice a copy of this section.