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from "Dave Plowman (News)" contains these words:


Would you say that using one as a spacer for (say) a deeper accessory is
legitimate?

Yes - although you'll have to modify it somewhat. They have normal 2.5mm
fixing threads - so if you wish to put it between accessory and backing
box you'll need to drill them out. You may also need to remove the spare
'lugs' as these can interfere with some fittings.

It will look a bodge, though.

For a start, the machine screws are M3.5 -- that is the normal variety
for electrical boxes

You're not usually expected to drill them out but rather to use one set
for mounting the spacer and the other set for mounting the accessory.
Of course that sometimes causes problems of orentation and fitting of
some accessories.