The first one is a michell part that appears all over star wars and you're right, thats the rear half of the V8 pulley! are there props out there that have survived?
I've always assumed that the blasters built on MG34s and Lewis Guns from the first movie got returned to their normal condition when they were returned to Bapty, which was after all in the business of renting prop guns to filmmakers. Sterlings would have been fairly common and relatively inexpensive, so the E-11s might not have been unconverted. But by 1976, MG34s are over 40 years old and would have been fairly valuable in their own right! I figured that's why those guns are so minimally disguised - to make it easy to restore them after.