I restored one just like it for my MIL. It was her grandmother's. The cable/cord is clamped at the end under the machine. There is a nut you can pull through more to adjust. Question, how does the board at the rear move? Or does it? The hinge system is the key to figuring this out. As everyone has stated, that board supports the head so the cable doesn't have to. I have some pics that may help. Let me know if you need them.
Sweet little machine. After I sewed on her, my National paled in comparison. It's like a Cadillac. Great investment!