you can get a 3/4 to 20mm adaptor. I have never in years come across a 3/4 male brass bush. I have known people ask for a 3/4 brass bush but actually mean a 20mm which is available.
Unlikely nowadays, but if not using it for Earth continuity, you could likely fit a 20mm socket over the thread and use a 20mm bush, once the box is on then its unlikely the socket will move.
Yes, the conduit is being used as the CPC. However some of the light switches don't have a bush. It is hard to tell if this was by "design" because the back box is not in contact with conduit, by a few mm. However the metal back box has plastic screw holes, so the face place is insulated, and the switch is double insulated as a result.
I can continue with plastic face plates etc, if I can get brass bushs of the correct size I can bring the CPC to the back box and the face plate, which I would prefer. In this instance there is not enough room for an adapter.
I haven't tried it myself [and to be honest I dont see how it could work] but I've seen suggestions that simply recutting the thread on a 20mm bush with a 3/4" die can work.
I've also heard of wrapping wire onto the 34" conduit thread then use a 20mm bush. again not something I've tried.
My thoughts are running now. The 2 threads are nearly similar and a 20mm bush almost feels like it's screwing onto a tube [I know as I've tried mixing]. So how about drilling and tapping a 20mm coupler and fitting a couple of grub screws, ti tighten against the 3/4" thread.
It's been a long time since I've been in this situation and I'm starting to doubt which tube diameter is the bigger 20mm or 3/4"
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