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Huh, I was wrong about where the ball joints are on Ultron. I thought they were at the thigh joints but nope, hip uses regular Revo joints.
The elbows uses the new special joints. The ball joint ends are at the lower arm side. You see a little bit of empty space there.
Makes sense since it seemed unlikely the ball joint could hold the weight of the entire leg.
Venome Snake should have these special joints pair too but don't seem to be the elbows...maybe shoulders?