Although the D78 Stock is that most usually associated with the District Line, it also operates the C Stock (as usually associated with the Circle and Hammersmith and City Lines) on its route between Wimbledon and Edgware Road.
There are actually two variants of the C Stock - the C69 and C77. But, apart from minor differences they cannot be told apart. Each is compatible with the other when forming trains. The latter batch was actually purchased to replace the old CO/CP Stocks which had been used by the District Line on the Wimbledon/Edgware Road service and as the D78 Stock cannot operate in passenger service between High Street Kensington and Edgware Road the decision was taken to build more of an existing type, rather than a completely new design being developed for the very few trains required.
They C and D Stocks are totally different trains - and whilst the D Stock is well liked by its drivers the same cannot be said for the C Stock!
With one or two minor exceptions all duties on C Stock trains are now carried out by either Earls Court of Acton Town drivers.