In the scene where Darth Vader is being "assembled", at the very end, right before the mask is put on, you see that there is a droid coming in carrying the mask & starts coming down with it towards Darth Vader. When you're looking at Anakin's face, you will notice that there is no neck piece for the mask to attach to. You can still see Anakin's burnt up neck, but no neck piece. In the next scene we see the inside of the mask & see it turn on, which is followed by the mask being hooked up to the neck piece, which wasn't there in the previous scene.