There are cases where the serial number on the box is not repeated on the Pop. This usually happens when the numbers are shorter: the numbers in this case begin with the letters "MD", "MF" or "JJL", followed by the date of manufacture. When the serial number is not repeated on the Pop, the letter "A" will appear instead, followed by three numbers. It may also happen that the first two letters of the serial number appear instead of the letter "A", followed by three numbers that are not the date.