That glass jar featured on top of early pumps? You would pump gas (often by hand) until it reached the desired level in the jar, which had a calibrated measuring stick inside. Then you would pay then empty the jar through a hose into your car or container.

The trick? (there's always a trick). The measuring stick took up volume, reducing how much gas you actually got by a tiny margin. Similar to the 'thumb scale' on the back of early butchers' scales (the butcher could put their thumb on the button, add an ounce or two to the displayed value) and charge for more than you were getting.

The gas jar measuring stick could have been just engraved lines on the jar of course, and taken no space at all.