I think what you are asking about is the little anchor nut in this image. check your bikeshop they should have one. you may have to buy the whole hanger but $3 isn't bad. it holds the derailuer in place when you take the wheel off. sometimes bikes had something on the other side too as an aid to center the wheel.
http://www.ebikestop.com/index_derai...ble-DP1071.php
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk