Below is the sole example of a fossil that I found in the Keyser formation near Mt. Union, PA. I think it's an Eospirifer sp. as the genera is known from the Helderberg fauna which is upper Silurian to lower Devonian.
The Keyser formation spans the boundary between the Silurian and Devonian (Pridoli to Lochkovian stage) and thus it is hard to date exactly when a fossil came from. Based on the stratigraphic location of the beds I was searching being closer to the underlying Tolonoway formation (Silurian, Pridoli stage) than the overlying Old Port formation (Devonian, Lochkovian stage), I am betting this fossil came from the very latest Silurian.