I have one of those in one of my many boxes of old stuff hidden away. As has been said it doesn't work very well, and the float and chain rattle around quite a bit, and the chain tends to get in knots and show E way early. The trick is to calibrate it so that it hits E just about when you need reserve, by adjusting the length of chain between the float and cap; one end should have a clip to allow this. This will show F when the tank is around half full though, there's no range adjustment and from F to E is only two inches of fuel height maximum. When the float is floating it and the loose chain rattle against the tank, and if things get bumpy the chain will get in knots.
Anyway, it looks sortof OK and works sortof almost. I used it for a week or three until the rattling and general uselessness inspired me to go back to the stocker. The trip meter has rarely failed me, 100 miles and it's time to find a gas station. Kids like to turn the little knob, though... reserve gets a lot of use too.
If you really like the look, disconnect the chain and your tank will always be full