Bob
Because I’m a bit of a cheapskate I made my own security bolt by pressing a stainless dowel pin into one of these (M12x70).
http://www.ebay.co.uk/itm/M12-12mm-A2-ST...=item1c4c8b1d65However here’s the real thing.
http://www.ebay.co.uk/itm/M12-x-80-Pin-H...=item3f401620e8My threaded coupling didn’t come from here but this is where you could acquire one. This seller will manufacture to your spec and they can produce internal metric threads (I have no connection).
http://www.ebay.co.uk/itm/THREADED-STUD-...=item4178a6ca53My coupling is 60mm long with a M12 and M10 thread 25mm deep. The M10 cap screw passes through the imperial threaded nut welded to the bracket (it doesn’t screw in). However perhaps an alternative is to swap the M10 internal thread in the coupling for a 1/2 BSW x 40mm long external (I’m 95% certain that the thread in the car is BSW – but check). That way you could then screw the coupling into the car and lock it solid with a nut to stop the coupling rotating (I use a small pin see my pic).
Obvious but don’t forget the coupling length is dependent on your tyre size. My spare is 165 wide and the wheels are 185. If your tyres are wider you need to add half the difference to the coupling length.