D for right (dinero), S for sinestro - left.I just looked at my spares, and they seem identical, however the pair I gave to a good friend of mine were marked. The drivers side was marked S for left, the passengers side was marked with the first letter of the Italian word for right. I forgot what it was.
The winders on my Duetto turn to the right on the passengers side to roll the window up, and to the left on the drivers side. The gears are the same it must be the way the cables are run that makes a difference. Classic Alfa also lists them left, and right.