If this plate is in the upper part of the door post I think I know what it is...
I have seen this once before.
It is a temperature sensor for your engines emissions system.
Ford experimented with a temperature sensitive ignition system in 72/73.
If your engine / emissions/ distributor are still stock you will find there are a few items that are different in comparison to a "normal" Bronco.
Very few of these trucks were built, even fewer made it to the open consumer market...