63 Dart Mystery Circuit Board Washer

I've been having a few intermittent issues with my 63 dart since I bought it last year but electrical is 100% new to me going into this and I'm needing a little advice.

I had the gauge cluster out to replace a few wires from the harness that were grounding and that I believed were causing my fuse for my taillights to continually blow and I had a washer I wasn't expecting to be there fall from the circuit board. Can anyone tell me where this guy is supposed to go back? I can't find any info or photos and it only seems to have one or two places it could go but I'm not in a hurry to mess something up.

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