This one through me off until I found your site some 2 years ago. For decades, I had known the diner/farmhouse at the northwest corner of Columbia Avenue and Running Pump Road had been known as the Three Mile House, so I just figured the milestone would have been there. It was at this time that I zeroed out my trip guage and measured the road mile by mile. From the site of Marker 65, one mile actually targeted an area about 700ft west of Running Pump Road at approximately 2341 Columbia Avenue. I would say this marker has been gone since at least 1973, as I now live less than 10 minutes walking distance from this marker site. With that said, I definitely would have noticed it sometime in a drive by. The coordinates are near 40.040751, -76.367087. In the attached photo, the stone could have been anywhere near, or back from the road. I even checked the other side of the road to make sure.
Tom K.