OK, it seems we have three different parts here.
1) "captive washer = unverlierbare Scheibe", which Leo already has. "Unverlierbarscheibe" (from dictindustry) may be added here as an odd alternative.
This is the freely rotating washer that is mounted under the head of a bolt or screw where it can't get lost.
2) "captive C-washer = Schwenkscheibe" is the C-shaped washer that functions as a hook.
dict.cc's definition was misleading here, sorry (http://www.dict.cc/?s=Schwenkscheibe
3) "swivel plate = Drehscheibe oder Schwenkplatte", which Leo already has. Larger flat swiveling device. Some pictures here:http://www.google.de/search?hl=de&safe=off&bi...
In any case, I think the combination "captive washer (w/o the C-) = Schwenkscheibe" is incorrect.