This device allows you to send the reading on a mechanical fuel gauge to a dashboard mounted electrical fuel gauge.
It attaches directly to the dial mechanism on the sight gauge on the tank sender (need to remove the glass cover), and converts the motion of the floating arm to electrical impulses at the standard 33-240 Ohms - so it can be used with any dash gauge.

Conversion is simple by removal of two screws
Sight gauge shown is for illustration; all you get is the black thing with the wires