They did a lot of research on the plane, so I would be surprised if they missed something like that. One sign that a prop can change pitch is the presence of counter-weights at the hub.
Here is a Dutch Fokker with variable pitch prop:
Here is a Danish one with fixed-pitch prop:
And Finnish:
http://upload.wikimedia.org/wikipedi...8FR-110%29.jpg
I think that someone at DT copied an existing D.XXI in a museum, as the cockpit model in the game matches it perfectly. That version probably had a variable pitch prop. So maybe, the Finnish cockpit represents the wrong one, but the Finns could have kept the extra levers....I don't know who is right, but the functionality of the Finnish D.XXIs is correct.