The 3 post gauge has the mechanical voltage limiter inside it. So your debris is/was carbon build up on nichrome windings. It came from that limiter, the fuel gauge, or more likely both.
I=input=12 volts. A=altered or adjusted voltage. Approx' 5 volts pulse fed to the fuel gauge internally and out to the other thermal gauge(s) via that post and circuit board(s). S=sender.
A to S at 20.7 is close enough. Check each post to the housing.
Did the other thermal gauges work like they should?
Footnote for the others reading here... 2 post gauges are using those posts marked I and S. The input was altered elsewhere.
