@circusdog - Good question
If you are referring to Indention A then:
The original inspiration comes from the idea of adding plates that would mount flush to the back that would slide around or fold out to "transform" between different configurations.
If you are referring to Indention B then they came from visually confirming the "duck tape" in the series footage from the Smithsonian reference shots I had found via google.
Just an FYI, I am NOT using any new-FX shots for reference.
I've included below some of my references and a close-up of impulse area on my Work In Progress: