I do not believe that the "DD" was ever used because the 6th Ave Subway, just like its El Predessor, mad all stops on 6th Ave.
The 6th Ave El was always a 2 track structure and for most of its existance, the subway had only two tracks in service and none of the stattions were ever constructed as local train stops only.
That was an interesting post about the "DD" running special as a local on the 8th Ave Line. The "CC' had always been the rush hour locals on the 8th Ave line, but I guess they had to run the "DD" as a non rush hour local on the 8th ave line. Is that true?
The "CC" terminal in the Bronx is Bedford Park.
The "DD" would have been 205th St with the "D".
The "C" train never ran when I rode the line in the 40's, 50's and 60's. So, I would have to assume the "C" terminal would have been Bedford Park also. The southern or Brooklyn terminals may have varied except for the "D" after the Culver connection, and then it was always Coney Island/the Stillwell Terminal.