I am betting on a property marker also, actually certain of it That is why the N20 marker is located 5 feet away. They always place the state markers nearby, usually with a line pointing toward the old stone. The question is what would the N stand for? In Wharton and Lebanon the state markers say NJ. I would assume Bass River might say BR, though I don't know because I have not looked for any there.
The real question is where is it located? Bass River State Forest is actually many different location scattered all around, some quite a distance from each other.
One thing to remember is that many of the older inscribed stones were actually placed there much later than the date on them. I know this because of talking to a property owner who informed me that a stone on their property with an old date, does not show up in records until many years later. All previous records don't mention it. In any event the one in the photo looks old.
If anyone comes up with this location I would be interested in visiting it.
Guy