This mark is located in Newton, Massachusetts, near the Green Line (D) between the Newton Highlands and Eliot stops. The mark is located on the north side of Route 9, along the sidewalk, at the northeast corner of Hartford Street and Route 9 near the bridge taking Route 9 over the MBTA tracks.
The stone itself is a few feet from the sidewalk. A homeowner's fence has been constructed near the roadstone, but goes behind it, so that it is accessible from the sidewalk. It is unclear whether it is on private property, but it is readily viewable from the sidewalk regardless, and the odd path of the fence would suggest that the right-of-way includes the roadstone.
The roadstone itself is quite weathered. The legible text reads:
To Boston
Line Ms
1810
Read below about the historical significance, per a informational brochure from Newton.