Yes, the part the other pieces are thraeded on to;
a screw-, stud-, boltlike thing, the outer part is threaded. It works independantly of the electricity. It's a relatively simple mechanism and the only action it has is a couple of metal parts connecting the presser bar lever to the mentioned split screw behind the faceplate. It pushes in a small
metal bit, moving inside the split screw; it again pushes the tension disks appart. It should be visible from behind the face plate.
Polish up the disks with metal polish or use fine steel wool on them. If it's literally rust they are steel and should stand up to it. They usually clean up fine. If they are aluminium be more careful, but aluminium doesn't rust so I guess they are not. I have used waxy polish and ended up washing them in hot soapy water after; I suddenly worried they might be too shiny and slippery for the thread. I remember I had all the shiny metal parts in hot water with machine dishwashing powder (a 201 I had was a bit grimy). I was more careful with the painted parts, dial with numbers.