25 Pa. Stat. § 294
The assessor or registry assessor making the enrollment, in districts other than in cities of the first, second and third classes, shall ascertain, if possible, at the time of making the annual assessment, by personal inquiry of the elector, the party politics, preferences or affiliations of the elector, and note and record the same, with the residence and other particulars required in making the assessment; and in case the elector is not at his residence at the time the said assessor or registry assessor visits the residence of the elector, he shall leave a certificate of enrollment which the elector may fill out and give to the assessor or registry assessor at some time prior to or on the sixty-second day preceding each primary, at which time said assessor or registry assessor shall sit at the polling place and receive said certificate; at which time the same shall be recorded by the assessor or registry assessor in the register.
25 P.S.§ 294