Without that part, the OP's derailleur is nothing but a paperweight or a donor of spare parts. There are two listed on ebay.
http://www.ebay.com/itm/VINTAGE-MAVI...item231a747426
and
http://www.ebay.com/itm/MAVIC-REAR-D...item4600d7dd7c
I have a good used one I'm willing to sell.
Last edited by Ecrevisse; 03-02-12 at 10:59 AM.