Need Help with Arri Matte Box

[Centwo]

I have this Arri Matte box

http://cgi.ebay.com/ARRIFLEX-MATTE-...lm_Cameras&hash=item5194a3b5d5#ht_4521wt_1136

But the back part was lost where the screw is that slide in to the matte box
what is this part called?
does any one now where i can get one that will fit this?