I think I have located the source of the problem, as the videos below will indicate. Anyone watching the videos might be able to spot the difference from a working Imperial Model B (black) and a non-working Model D (green).
I have removed the keyboards-typebaskets on the Model D and my Model B to see where the differences in the mechanisms are. And the inner workings of the Model B and the Model D do differ somewhat, as can be seen from the photos below. One obvious thing is that the B has a rounded, fairly wide plate under the typebasket which activates the pawl and the carriage movement.