Oh, the plot thickens. Well, not a plot per se but the possibilities. I bet you knew that New Jersey before it was the glorious NJ we know today was split in two? Three different lines over a short period of time were used to split the land into East and West Jersey. But did you know the land was given by the Duke of York in 1664 to Sir George Carteret and John Berkeley, 1st Baron Berkeley of Stratton. Carteret got the West and Berkeley got the East (least how I read it on Wikipedia).
So is the marker with the letters JB one of the disputed lines dividing the state into East and West Jersey? JB for John Berkeley the 1st Baron Berkeley of Stratton who from all accounts was full of himself. The Thornton Line is a boundary line or partition line surveyed in 1696 that runs nearest the area of this marker.
Thorton Line- https://en.wikipedia.org/wiki/Thornton_Line#/map/0