The message from "Dave Plowman (News)" contains these words:
Perhaps there's a bit of generalisation going on here.
Are ALL plastic accessories exactly the same dimensions?
Are the dimensions of all metal accessories identical to each other and to those of plastic accessories?
Are the flanges of all dry-partition boxes identical in dimensions and in relation to the accessory-fixing M3.5 machine screws?
Is there a rebade on the back of all accessories sufficient to allow the accessory to fit OVER the flange and lie flat against the wall surface?
Clearly many accessories do fit over the flange. Others may not.