What is the difference between what is referred to as a C-Model and a D-Model? Technic sets often have an A & B model and I thought a C-model was a fan created alternate option, which leaves me wondering what D-model is.
For instance, I'm pretty sure the Duck model I found online that is built using only parts from 42040 is considered a C-model. (Fun model to build, though one of the 12 tooth gears doesn't like to stay put for long)