WM 6453: What is the extra piece of metal that is included in the box for?

The extra piece of metal is a VESA extension which gives you the possibility to install a device with the VESA size 400x400. The smaller, already mounted piece is made for the VESA size 400x200.

To mount the extension, please refer to the manual of the WM 6453 TV wall mount, from step 5b onwards.