QuoteIts a great shot but its not the filmed shot. Scott wanted to make the Alien otherworkdy, hence the crab wslk shot
You can see the camera in the lower left corner of the shot with the water cover over the lens.
He made it otherworldly by filming it at a weird angle that made its exact shape hard to distinguish -- hence your confusion.
But that's a foot.
There was no special rig made. What you have outlined is the foot planted on the landing gear while the rest of the body lowers further.
You wanted to know what it was, that's the actual answer. You can keep saying everyone else is wrong, but that's all there is.
PS: It's not Bolaji in the suit. Bolaji didn't like heights and refused to do the shots for the final scene. Powell is in the suit (as above!)