Today, cans are pressurized. Vintage metal cans were also pressurized. The difference, is that early metal cans were often lined with a coating of lacquer. The balls were placed inside, all air was withdrawn creating a vacuum. Pressure was then shot into the can to exactly balance the pressure inside the ball. There are some cans that are labeled “vacuum packed”. This was either a marketing error, or they were referring to the above mentioned process.