I'd like to add at this point that beginning with Marker 64, there was likely a third measurement, that being 1 to L between the milage 64 to P and 9 to C engravings. It would have been 2 miles at 65 and 3 miles at 66 also, as the two surviving markers, 68 and 71, as well as a photo of Missing Marker 70, show the 3 measurements.
As close as I could tell, this next marker going west would have been at approximately 40.041473, -76.388129. I tried to get an accurate measurement by backing up one mile from Marker 68, which still exists. The attached image shows the site within a few feet either way.
Tom Kipphorn