"Street line marker"?

[Pete from Boston]

Found this in the sidewalk in Rockport, Mass., but posting in General because I find so little local info that I wonder if there are other examples.



What's the story with this?  Obviously some kind of right-of-way marker, but to what end?

Essex County no longer exists other than as a line on a map, a court system, and a pension plan, so there's not really someone to ask directly.