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Back_spine_pt01 und Back_spine_pt02 should be soldered to back_half_pt03 and back_half_pt04. Maybe a bit into back_half_pt02. I would first Finish both halfs of the back and then connect the spine.
The part Back_spine_pt01 was connected with flexible strips to the hips. for a bit more movement. I used it 2 times, but you can also use it a bit more often if you are bigger than me ;-).
I hope this might help you.