What Andurp said. If you didn't already own one before the cutoff date you are S.O.L. Back in the day, FNs were at one time non-restricted, then restricted after '78, then finally prohib.
I bought mine when they were restricted, for target shooting. After the loss of the SAPs for taking them to the range, I had to register as a collector before they would transfer any more of them to me.
Even being grandfathered can suck. Back in the day we were told that "If you have a prohibited firearm you will be grandfathered for prohibited firearms". They had not at that time made the various classes of prohibs yet (except for F/A).
Being a poor starving OCdt at the time, I didn't have the cash to buy more prohibs than the first L1A1 that I bought. I passed up a C/A Bren gun and a Makarov pistol. I didn't feel too bad, because I was sure that in the future, when I had more money, I could get them, because after all, I would be grandfathered for prohibited firearms. Imagine my surprise and dismay when they came out with the various classes, and I discovered that I would only be 12.5.