There are black and gray plate triodes for each of the big and little halves, so there are:
big triode black, small triode black
big triode gray, small triode gray
big triode gray, small triode black
big triode black, small triode gray
There are also series/parallel heater arrangements, though you'd be hard pressed to hear a difference there.
There are also coin based 6DN7's that are built a little differently.