72 Pa. Stat. § 5951c
Whenever any tax collector or receiver of taxes in any county of the sixth class shall have failed or neglected to make return to the county commissioners of taxes assessed and levied against seated lands for the years one thousand nine hundred and thirty-eight, one thousand nine hundred and thirty-nine or one thousand nine hundred and forty, within the time required by law, the tax collector or receiver may legally and validly make return of such taxes in the manner provided by law for such return, at any time within six months after this act takes effect, and the liens of any taxes for said years so returned under the authority of this act, shall be valid and are hereby ratified and confirmed, and a county treasurer's sale may be had for such taxes at any time when a treasurer's sale is held, or on any day to which said sale may be adjourned or readjourned in the manner provided by existing law: Provided, however, That the return of any tax under this act shall not revive or re-establish any tax lien against real estate which was transferred to any bona fide purchaser during the time when any such lien was lost, nor shall such return affect the priority of the lien of any mortgage or other lien which was entered prior to the tax lien, or which gained priority because of the failure of the collector to return the tax at the regular time.
72 P.S. § 5951c