Lucas 318346

You will also require the small rubber unit that goes into the switch, where the key inserts. 

If the switch is well used/abused then the thin metal construction inside the 'small rubber unit' that holds the correct key in place will be well 'mullered' so a new on of them as well.
While talking about the PRS (see my web site for its servicing) The screw that holds the knob on-when Lucas made this they inserted a (6BA) screw down the centre of the knob retention screw to spread it out and stop the knob rattling. All subsequent screws have NOT had this inner screw.