I think the sign you are referring to is not this one. They are very close though. You can't see this sign from any specific road. And the stone you are thinking of I believe is concrete. I was looking at it today. If I am wrong on the stone I will have to check on it later but I don't think so.
Getting back to my photo of the sign, the location of the sign in my photo above is 175 feet from the monument in the photo above that you mentioned. The next time you are at that monument walk around in that area, staying on dirt and not entering the woods and look for it. You will be surprised when and if you find it. Be warned, NJ71 is even further down the road and you may be thinking of that one.
I just realized you are thinking of NJ71 on the right side. Just so you know NJ70 is located at this link on the left side of the road headed away from Allens Road. The sign is 175 feet away that you need to find.
Yes guy I just checked the photos on my phone and the marker I am referencing is nj 71. I haven't figured any easy way up uploading to the site from my phone. Next time I am in the area I will look for the sign.