"As a guess, I would say you have stumbled on one of the markers from Beach Haven to the Upper Delaware, marking the division between East Jersey and West Jersey. As it happens, I have a regular monthly meeting with the Surveyor General, and will ask him. Sometimes he can't make it, so keep me reminded if I don't write back. Did you photograph it?"
Wolfspider,
Unfortunately, the WP1700 stone is not one of the stone for the division line between between East and West Jersey. That was the Lawrence Line, and I have visited or found all of the known stones from this line. The Lawrence Line goes from Egg Harbor through the Warren Grove area and up through Ft. Dix.
Guy