Joe - that's the "C"-clip that's under the handle nut and its purpose is to take up any clearance in the bronze bridge sleeve-to-brake plate fit.
If you take off the handle nut and look at the Stainless end of the brake-plate spindle protruding up through bronze sleeve you will see the incised groove that it fits into.
Its made of stainless and is almost impossible to find in any hardware store, but any decent tackle shop should have them to fit most any Abu.
If you want, PM me your mailing address, I have a bunch here at home and I'll send ya a couple.
(This post edited by Leprechaun on 03/01/2003)