The white lever on the right is a hand pump. You toggle that back and
forth and it pumps gas from the storage tank up into the glass chamber.
The glass chamber is the measuring device. You keep pumping until the
glass container contains as many gallons as you want, as indicated on
the scale. You then open the valve to the hose and the gas gravity
feeds from the glass container into you car or whatever.
The picture you linked has the numbers running zero to ten from the top
to the bottom. In this style you are supposed to fill the glass chamber
to the top, then draw out as many gallons as you want. Of course if you
only want four gallons you can save yourself some pumping and only fill
the container to the six mark, then drain it to the ten mark. (Back
then people needed better logic and arithmetic skills than we need
today.)
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu