the dust cover is the forward extension of the frame that mates to the slide when the gun is in battery. A commander frame has a shorter extension (due to the shorter slide) so mounting a full size slide on that frame will result in a gap between the front of the frame and the back of the slide on the bottom of the assembly. Since the Commander frame is otherwise identical to the 1911, I fail to see the advantage of this other than saving buying another gun.
the dust cover is the part of the frame in front of the trigger guard where the recoil spring/ guide rod resides, it is shorter on Commander models than Guvmint models