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.