It's called the "Fuel Pacer" and lots of Mopars had them well into the 80's. I've seen minivans with a green LED labelled "Fuel Pacer" in the fuel gauge.
I had a Valiant Brougham with it. Really neat gadget. Started many conversations especially at night. With the 2-barrel on the engine, that light was on constantly.

For poops and grins, I used a fuel pacer switch in my baby blue 6-cylinder Hornet to illuminate a "Fuel Economy" lens in the gauge cluster.

Kinda hard to see here but it's the yellow lettering in the lower left quadrant of the warning lamps. I got the lens out of a Pacer cluster as some AMC Pacer's had a similar Fuel Pacer setup.

I have one NOS Fuel Pacer switch left. Don't know what I'm going to do with it yet. I had thought of using it to turn on some strip LED's in the cowl induction hood on my Javelin. That way, at night when you get on the go-pedal, the cowl induction will light up orange.