Early ones had an assembly number. Later ones had the serial number. Can't tell you when the change was made. Also can't tell you when they stopped altogether with the numbers. I only have two that I can check: 145XXSA (1957) has the same number as the number on the loading gate (assy number) and 244XXSA (1959) has the last 3 digits of the serial number. Both are on the front face of the cylinder with a number between each chamber. Perhaps other members with 2nd generation SAAs could check and see how their's are numbered.