Those are simply I.C.C.S.'s Certificate # where they have their own record keeping system of the coin. The coin is now registered with them and is entered into the Population Report.
They (not long ago) once were at 5 digit Certificate #'s, however, I suspected soon they would move to a 6 digit since all the alphabets they used were completed.
I could never understand why T.P.G. companies simply don't use the date as part of their Certificate #? ie: for example: The start of every year on January 1st, the serial # would read 2012-, the start of every month would read: 2012-01, and the day the coin would be graded would be 2012-01-01 and each coin graded on that particular day would read 2012-01-01-01 and consecutively there on.....
This way any owner of the coin would know almost to the hour when the coin was slabbed and T.P.G's would never have to worry about duplication of #'s!