It is a piece of spring steel resembling a clip that would slide onto the edge of a piece of sheet metal inside the door....if I remember right. If I had one, it would be yours.
Good Luck.
This is the bracket that held the cable out of the way on my '70. However, when I went to reassemble I could not, for the life of me, find the mounting point inside the door. I ended up tying it out of the way in a couple of spots.
On the front part of the door.If you unscrew the three screws that hold the weather strip to the door you will see a large hole.That is how the bracket is secured to the door.