I am at a loss trying to figure out something very basic!

I have the holdown that I have never used. It is two pieces; one flat bar with a small hole in the middle and another flat bar with the ends bent to 90 degrees and a threaded receptor in its middle and there is a thumb screw. There are two slots in the body of the tractor where the two batteries adjoin but the slots are a couple inches above the top of the batteries. Hopefully you know what I am describing. Not too complicated. 3 pieces! I can't put the puzzle together . Am I missing an important 4th piece?

Mike