Help - wiring question

[ScottH]

I noticed today that when I was trying to trace the wiring in my 72, there appears to be a piece missing. It is a small silver box located above the fuse box on the inside. Normally I believe there are two, but the top one in my truck is missing.

If anyone can tell me what this is normally used for I would appreciate it. I have a parts truck with one on it still, but want to be sure it is needed before I start rewiring my driver inorder to replace it.

Any info is really appreciated.

Thanks!