Te pongo lo que he encontrado en google:
Not all maestro cards have an issue number. Count the number of digits in the main card number (across the centre of the card). If it is 16 digits, there is no issue number. If it is longer, there is.
If it has an issue number it will be on the front of the card, near the expiry date. It will simply say 'issue' followed by a one or two digit number.
If there is not an issue number, you will need to provide the 'valid from' date instead.