Here is the main difference between the HR and some earlier ignition barrels, the HR needs the barrel removed from the assembly before you can remove the nut that allows you to remove the assembly from the back. Some of the earlier models allowed you to remove the nut without removing the barrel and you could therefore drop it all out and do this job out of the car. The reason for the HR being like this is as a basic theft deterant, if you can remove the nut easily then you can replace the whole assembly easily with your own and Mr Thief just drove off in your classic, luckily your undamaged ignition assembly will likely be laying where you parked your car ;)