I'm not sure how commonly this is known, but did you realize that every one of the 67 counties in Pennsylvania, in addition to being home to many other markers, has a PHMC marker of its very own? I've been trying to figure out the best way to talk about these, but as is so often the case, I'm probably overthinking the matter and I'm just going to treat them like the other markers.
The marker for each county is found in the vicinity of the county courthouse, and the community where the county courthouse is located is known as the county seat. (The lone exception to this rule is Philadelphia County, which consists entirely of the city of Philadelphia - it is its own county seat.) So if you haven't had occasion to wander past the courthouse of a given county, odds are you've never seen one of these markers.