Having owned a transferable FNC back in 2003, I know a little bit about these fine rifles. Respectfully it shares very little with the FAL which I also own.
Additionally, the Gun Control Act of 1968 barred importation of foreign made full auto weapons for civilian consumption. Given that the FNC was first produced in 1979, I'd say the chance of you having a genuine factory gun is slim to none.
http://en.wikipedia.org/wiki/Gun_Control_Act_of_1968
Who's listed on the F-3 as the manufacturer?
Who's listed on the gun as the importer on the rifle?
Howco, Gun South, ?
Does it have the three round burst installed?
My FNC was a Curtis Higgins converted sear gun which are the most common. Bill Fleming produced a few registered receivers using correct factory parts.