72 Pa. Stat. § 5971a
From and after the passage of this act, it shall be the duty of each receiver or collector of any county, city, borough, town, township, school district, poor district or institution district taxes, to make a return to the county commissioners of such taxes which are assessed and levied on seated lands, and which are unpaid, and for which no liens have been filed, not later than the first Monday of May, in the year succeeding the year in which the respective taxes were assessed and levied, filing therewith a description of said real estate by adjoiners or otherwise, sufficient to identify said land with name of the owner or reputed owner against whom such taxes were assessed and levied, and the amounts of taxes unpaid, and the year during which the said taxes were assessed or levied. In addition to penalties under existing laws, interest at the rate of six per centum per annum shall be added, beginning on the first day of May, of the year following the levy and assessment of such tax, until such taxes are paid or the seated lands responsible therefor are sold as hereinafter provided.
Whenever in this act the words land, seated lands, real estate or property are used, such words shall be construed to include a mobilehome or house trailer subject to a real property tax.
Where the receiver or collector of any such taxes shall have failed to make the return thereof as provided by this section, and the time fixed for making such return has expired, such receiver or collector of said taxes, or any delinquent tax collector, or taxing authority in whose hands such taxes now repose for collection is hereby authorized to make return of any of said taxes which are unpaid and for which no lien has been filed within six months from the effective date of this amending act; and the liens of any taxes so returned shall be valid and are hereby ratified and confirmed, and a county treasurer's sale may be had for such taxes at the time when a county treasurer's sale is held under the provisions of this act, in the same manner as if such returns had been made at the time heretofore required by this section: Provided, however, That the return of any tax under this amendment shall not establish, revive or re-establish any tax lien against real estate which was transferred to any bona fide purchaser during the time before such return was made or when any such lien was lost.
72 P.S. § 5971a