Beaver Co. PA markers 

There are only two possibilities for the marker you showed me: it is either a local marker or the PHMC database is incomplete. After combing through the PHMC database, I have to say it seemed very up-to-date and complete, even listing very old missing markers.The shape and design of that marker suggests it is either not a PA state historical marker or it is a very old one. My guess is that it would be a local marker.