Nut securing PRS8 switch onto headlamp

I can't seem to find a nut that fits the prs8 ignition switch. Nut that holds it to the headlamp.

If someone knows the size and thread type, I might try 3d printing one. Help would be much appreciated

Thanks

Lucas usually used BA thread forms. This is kind of a metric variant. In relatively unstressed applications metric is usually close enough. On the other hand I have had some modern "official" Lucas stuff that has simply unrecognizable fasteners.