Flat plates are wider and will not fit on a dual box. Opinion on brands varies.
However i went to a certain manufacturers factory a few weeks back and for some bizarre reason they where helping another manufacturer pack their products!!!!
They look like Galvanized threaded couplers between the boxes,
They are made like that with a 20mm internal thread.
You then screw a Male brass bush into each end.
Probably about 40mm wide they are.
Actually, the screws for the middle accessory should have their slots horizontal, i.e. at 90° to the others, to reflect the fact that they are at the top and bottom of the plate rather than at the sides.
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