Plate with battery leads connected to it?

My name is Spencer and I am 18 years old. I recently finished swapping my 727 for an A-833 in my 1974 plymouth duster 360. everything was great until she left me stranded, the wiring harness was just a mess. Apparently, some electrical engineer got ahold of her a few years ago and filled her with some really weird... creations... i managed to figure out most of it but there's one thing i just cant wrap my mind around. I was Hoping someone might know what this is. I'm in the middle of installing an American autowire harness and so far, its been great. Just trying to figure out if i need this or what this even is. Any help would be much appreciated.

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