Thanks Pat. Yes you're right, they seem to go in from the outside, my mistake, I see how they work now. Regarding the plate, I think it would work fine without needing to be welded as these nuts would hold it in place, it follows the contours of the door skin nicely.
But there is no opening to insert the plate, I'd have to make a large hole in the back of the door to slide it in somehow. I'll probably not bother with the plate at all, I guess they were produced as a fix after the heavier mirrors started falling off, so there probably wasn't an opening for them originally.