It's about the size of your thumbnail, made of chequered steel and trapezoid in form, with a slot.

You slacken the forward rail screw, slip it in and tighten it up.

Seems a few were missed in assembly. Mine included.

It's free issue from the importer

try an email to:
info@carl-walther.de

Include your serial number.
Ask for a catologue as well.