Not a "Keystone" shape, yet on a typical keystone marker post. Marker notes it was provided by the Business Men's Association, perhaps as part of a program in cooperation with the old Highways Dept. A newspaper article stated that four of these were installed - a second at the Oakmont bridge, and a third at the Thornburg Viaduct, and the fourth near Crafton Heights. Only this one remains to our knowledge.
This is a variant marker, similar to “Borough Line” markers found elsewhere. Photos below courtesy of John Krakoff.
August 2023 photo by C. Boehm
PA Route 60 (Noblestown Road at Ridenour Avenue