The gallon cans were used by owners or dealers to verify pump output.There are five gallons ones used by state or county or even city weight and measure departments.The five gallon ones are more accurate because of larger amount.These are still used today.The inspectors would come in and test each pump,if it passed it gets an inspection sticker if it fails the pump has to be shut down till its fixed,It was not a good day when the inspector came,the last one I remember in the 80's cost $10.00 per pump.