It’s just an alternative back bracket. Some UPVC and older wooden doors have extruding mouldings which inhibit the default bracket fitted to the LetterBoxer.
Also, some older doors have very little room above the letter plate which makes it difficult to install with the default bracket. This alternative bracket enables you to fit the LetterBoxer in these circumstances.
We have included an Alan key in the envelope that came in the box so you can swap out the back bracket with this extra one. You can also use this key to adjust the fabric side panel lugs in case they come loose down the road. So, keep it safe.