ORIGINAL: Durian
There seems to be a number of people getting stuck at that part of the installation.
If you have a look carefully in your box, you find a little clear bag with that part, together with shims etc.
Seems to be like separate from everything else, kind to be like a after thought and they just chuck that in.
QFT
Your
should have the part, just look for a little separate bag. My friends savage was missing this part, and it was thrown in as an afterthought. However, when I bought mine (just a slightly newer revision), mine came in the bag it was supposed to. But yeah, look for a little bag, the washer should be there.