Not a lawyer but it would seem to me if you had a magazine that had no 9/94 or higher date on it and happens to have an FN Floorplate on it, the burden of proof is on the BATF. It's the manufacturer who was wrong and you acted in good faith. I seriously doubt that any BATF agent would sift through the magazines in my bag at the range and happen to notice an FN magazine. If he did, I highly doubt he's got some list of lettering style, finish style, or distinguishing characteristics that would lead him to believe he'd get away from stealing it from me!
I'd heard this about some DPMS magazines but don't know where either. As I have no DPMS or FN magazines, I'm not materially effected.