I don't know where the stone was, but I can find out. I might be able to plot the lat/lon this weekend.
http://westjersey.org/wj_line.htm This is all I've ever found of it. I never heard of this line before I got to chatting to the guy with the stone. Rt-571 from Cassville to South Stump Tavern Rd runs alone this line. (for the most part)
If you go to your link you we see that the Mahacomack line was proposed but never made. I suspect the stone you are referring to was the Hanover 1856 stone.
This stone was mentioned in the 1856 survey of John and Joshua Horner's property. It was marked with the letters and figures of "Hanover." It was placed there on 6/2/1856 in the presence of Joseph P. Jones, Joshua Horner, Moses Horner, surveyors George Sykes, Joseph Cox, and Joseph DeBow, Caleb Sykes, Freedon Horner, Joshua Horner Jr., James Horner, John Green and 2 other men from Hanover.
When I could not find it I contacted the man running Colliers Mills and he told me there were photo's of it oin his office. He had heard from someone that they also could not find it so he assumed it was gone.
If you can find any info on that stone I would be interested. If you know or find out where it is please don't post that info on this site. Contact me if you want and let me know where it is.
Guy