Hello and welcome to the fora, AW.
The mask ID is a code that is 1 number, 1 letter, 2 numbers and 1 letter.
The first number is a revision number of the 4 character mask code behind it. Not all revisions are released!
There is usually also another code which is usually 4 letters then 4 numbers.
I don't know what the 4 letters represent but the first two numbers are the last two digits of the year and the last two numbers are the week number within that year.
Freescale seem to like keeping this information all a bit of a secret.