>>23447015
by and large backpacks are described as detachable - even if going the extra mile to make them interchangeable is more than the usual, it's pretty common for them to be described as a separate unit distinct from the torso itself
there was a good graph on that in MS Bible with some more info, trying to hunt it down right now
regarding the grapple arm, the graphs make it look like the empty space between the backpack and back is pretty miniscule once the arm is contracted, functionally close enough that on a model kit you can just attach it directly onto the backpack