The question is whether or not NJ grandfathers weapons. That is to say if a weapon which is now legal to have in this state with a FID but,was purchased before the gun control act was passed must a FID be obtained in order to exercise the rights of possession and use of the weapon or is the weapon exempt because obtained before such rules were put into place ?

It is not a FID, it is a FPID - the "P" stands for purchaser. You are not required to have a FPID to possess or use a firearm in NJ.

NJ doesn't grandfather anything. The pending .50cal ban is a perfect example......

As long as the firearms were purchased (or otherwise acquired) legally and are legal to own currently (are not banned as "assault weapons"), they can be owned and used within the exemptions that NJ allows for firearms (keeping in mind NJ law makes all firearms illegal except under certain exemptions), as long as the owner is not a prohibited person.