I believe it's a cover for a pulley and drive belt. Most likely bigger components than those used on the first prop.   There is some discussion about it here:  

I wanted to ask about this extra cover piece on the dome, to the side of the pivot. These images show what I'm talking about. The pivot under the cowl looks like it has some kind of cover next to it. I remember reading somewhere that there was a manually controlled dome that was sometimes used. Is that what this is? I can only find one or two pictures, and even other pictures of this specific member of the Cult of Skaro (I think it's Thay?) seems to be missing this cover piece.  

Hi all,   What size do you do the neck rings? (inside and outside diameter in imperial)    Thank you!