Ah right. From what I've seen and the Landrover parts list the rear step bumper has a plate with 4 pre-drilled holes that you bolt a tow ball to, that could be a 50mm or 2" ball.
The difference in part numbers seems to indicate that 90s and 110s have different rear crossmembers and therefore different mounting points - plus the 110 rear crossmember changed with the 99 model year.
As long as the mounting points matched the crossmember I can't see that UK spec or NAS spec would make much difference, except for the UK where approvals for towing equipment neccessitate the correct stamp / sticker.