According to the supplier's information, the bracket with the holes on the side is shipped with this product. The rear pictures with the slots show a different product, at least they do not correspond to the information provided by the supplier.
Thank you for pointing this out, I will pass this on internally to the relevant department and hope to have it checked as soon as possible.

I don't know which slot you mean. It may be a misunderstanding. The bracket on which the box sits has a round base. This has a slotted hole (is that what you mean?) so that you can adjust the angle of the box. You can see it clearly in the first picture.
It is fastened with screws.