I’ve never seen that feature on any screen used T1 or T2 endo arm. I will look again to see if I can find it in some of my newer ref. But again, I want to state that my personal opinion on these particular items is that neither of them were ever actually seen on screen.
I do suspect they were made from molds that someone made from T1/T2 parts. But, they were cast in metal by someone and assembled improperly in some areas. I mean, the arm on the right has mostly T2 parts with some T1 pistons in the hand area. The one on the left I believe was made using molds taken from the real T1 hand/arm (maybe the same mold as the Bob Burns items?) Still, I don’t believe it is a production used item. I certainly cannot find a single frame of this pitted casting in the film.
However, I’d love for someone to prove me wrong. I was saddened to see it sold at auction as if it were screen used. So, I would caution using anything about these as reference for what the overall arm/hand should look like. That said, they do have some aspects that have helped clarify some missing information that can’t be clearly seen through film still frames.
-Dana