My amateur's opinion:
It looks to me that it has a sort of swiveling dish-like structure inside the 'mouth' that lights up when it fires. However, there are scenes and photos that show no 'dish' and only a sort of hole where the dish structure would presumably plug in.
I think it might be an optional part, or something that was originally there, but was lost or broken over time. IIRC, the wing up/down mechanism also stopped working eventually.
Comparing the pic Eric posted with my pics, it seems like that may be an entirely different gizmo than the 'other' one. I don't seem to see any areas where the orange 'rectangles' texture might show, or be illuminated. It also seems too 'domed' to me. If I had to guess, I'd say that was a prototype or replacement part.
All totally guesses and opinion.