Bill can probably clarify it, but as far as I know, the 'C' groups were the reserve formations for each Squadron, and would be assigned, or 'slot in', in elements, to a particular group in that squadron, as required, should the need arise. Therefore the specific elements would assume the call sign of the aircraft they replaced, for example, Charlie (group) One and Two, might become Whippet Three and Four, in the 61st FS.