None of the local Repco's had the right switch, ended up at Cheapa Auto. The part number is BS31.
There was no nut on the switch in my car. Considered that it may have been left off during the build. No nut comes with the replacement switch so I wandered along to a wrecker and the XE they had didn't have one either. Saw a Fairlane of the right vintage parked in a Nerang factory complex so found the guy who owned it and dragged him out so I could look under the dash...no nut either.
Anyway I agree that a lock nut would be a good idea so have installed the new switch with nut. The switch mount in the pedal box has an internal thread.
Old switch on top, new one below. Note thread damage on old unit which may have been the result of fretting in the pedal box mount.
Job done
Cheers