On The USA imported models, the FS screw is welded on. You will need to cut the welds to allow the nut/FS to be spun off.
Before installing the New USGI FS nut, lube the treads and spin it on/off the threads to mate the metric to inch threads a few times.
P.S. Don' forget to remove the front allen retainer set screw before trying to spin the FS Nut.